docs: document Claude Code OAuth concurrency model and external flock rationale (#637)

## Summary

Adds `docs/CLAUDE-AUTH-CONCURRENCY.md` documenting why the external `flock` on `${HOME}/.claude/session.lock` in `lib/agent-sdk.sh` is load-bearing rather than belt-and-suspenders, and provides a decision matrix for adding new containers that run Claude Code.

Pure docs change. No code touched.

## Why

The factory runs N+1 concurrent Claude Code processes across containers (`disinto-agents` plus every transient container spawned by `docker/edge/dispatcher.sh`), all sharing `~/.claude` via bind mount. The historical "agents losing auth, frequent re-logins" issue that motivated the original `session.lock` flock is the OAuth refresh race — and the flock is the only thing currently protecting against it.

A reasonable assumption when looking at Claude Code is that its internal `proper-lockfile.lock(claudeDir)` (in `src/utils/auth.ts:1491` of the leaked TS source) handles the refresh race, making the external flock redundant. **It does not**, in our specific bind-mount layout. Empirically verified:

- `proper-lockfile` defaults to `<target>.lock` as a sibling file when no `lockfilePath` is given
- For `claudeDir = /home/agent/.claude`, the lock lands at `/home/agent/.claude.lock`
- `/home/agent/` is **not** bind-mounted in our setup — it is the container's local overlay filesystem
- Each container creates its own private `.claude.lock`, none shared
- Cross-container OAuth refresh race is therefore unprotected by Claude Code's internal lock

The external flock works because the lock file path `${HOME}/.claude/session.lock` is **inside** the bind-mounted directory, so all containers see the same inode.

This came up during design discussion of the chat container in #623, where the temptation was to mount the existing `~/.claude` and skip the external flock for interactive responsiveness. The doc captures the analysis so future implementers don't take that shortcut.

## Changes

- New file: `docs/CLAUDE-AUTH-CONCURRENCY.md` (~135 lines): rationale, empirical evidence, decision matrix for new containers, pointer to the upstream fix
- `lib/AGENTS.md`: one-line **Concurrency** addendum to the `lib/agent-sdk.sh` row pointing at the new doc

## Test plan

- [ ] Markdown renders correctly in Forgejo
- [ ] Relative link from `lib/AGENTS.md` to `docs/CLAUDE-AUTH-CONCURRENCY.md` resolves (`../docs/CLAUDE-AUTH-CONCURRENCY.md`)
- [ ] Code references in the doc still match the current state of `lib/agent-sdk.sh:139,144` and `docker/agents/entrypoint.sh:119-125`

## Refs

- #623 — chat container, the issue this analysis was driven by; #623 has a comment with the same analysis pointing back here once merged

Co-authored-by: Claude <noreply@anthropic.com>
Reviewed-on: #637
Co-authored-by: dev-bot <dev-bot@disinto.local>
Co-committed-by: dev-bot <dev-bot@disinto.local>

lib/AGENTS.md

@@ -24,7 +24,7 @@ sourced as needed.
 | `lib/issue-lifecycle.sh` | Reusable issue lifecycle library: `issue_claim()` (add in-progress, remove backlog), `issue_release()` (remove in-progress, add backlog), `issue_block()` (post diagnostic comment with secret redaction, add blocked label), `issue_close()`, `issue_check_deps()` (parse deps, check transitive closure; sets `_ISSUE_BLOCKED_BY`, `_ISSUE_SUGGESTION`), `issue_suggest_next()` (find next unblocked backlog issue; sets `_ISSUE_NEXT`), `issue_post_refusal()` (structured refusal comment with dedup). Label IDs cached in globals on first lookup. Sources `lib/secret-scan.sh`. | dev-agent.sh (future) |
 | `lib/vault.sh` | **Vault PR helper** — create vault action PRs on ops repo via Forgejo API (works from containers without SSH). `vault_request <action_id> <toml_content>` validates TOML (using `validate_vault_action` from `vault/vault-env.sh`), creates branch `vault/<action-id>`, writes `vault/actions/<action-id>.toml`, creates PR targeting `main` with title `vault: <action-id>` and body from context field, returns PR number. Idempotent: if PR exists, returns existing number. **Low-tier bypass**: if the action's `blast_radius` classifies as `low` (via `vault/classify.sh`), `vault_request` calls `_vault_commit_direct()` which commits directly to ops `main` using `FORGE_ADMIN_TOKEN` — no PR, no approval wait. Returns `0` (not a PR number) for direct commits. Requires `FORGE_TOKEN`, `FORGE_ADMIN_TOKEN` (low-tier only), `FORGE_URL`, `FORGE_REPO`, `FORGE_OPS_REPO`. Uses the calling agent's own token (saves/restores `FORGE_TOKEN` around sourcing `vault-env.sh`), so approval workflow respects individual agent identities. | dev-agent (vault actions), future vault dispatcher |
 | `lib/branch-protection.sh` | Branch protection helpers for Forgejo repos. `setup_vault_branch_protection()` — configures admin-only merge protection on main (require 1 approval, restrict merge to admin role, block direct pushes). `setup_profile_branch_protection()` — same protection for `.profile` repos. `verify_branch_protection()` — checks protection is correctly configured. `remove_branch_protection()` — removes protection (cleanup/testing). Handles race condition after initial push: retries with backoff if Forgejo hasn't processed the branch yet. Requires `FORGE_TOKEN`, `FORGE_URL`, `FORGE_OPS_REPO`. | bin/disinto (hire-an-agent) |
-| `lib/agent-sdk.sh` | `agent_run([--resume SESSION_ID] [--worktree DIR] PROMPT)` — one-shot `claude -p` invocation with session persistence. Saves session ID to `SID_FILE`, reads it back on resume. `agent_recover_session()` — restore previous session ID from `SID_FILE` on startup. **Nudge guard**: skips nudge injection if the worktree is clean and no push is expected, preventing spurious re-invocations. Callers must define `SID_FILE`, `LOGFILE`, and `log()` before sourcing. | formula-driven agents (dev-agent, planner-run, predictor-run, gardener-run) |
+| `lib/agent-sdk.sh` | `agent_run([--resume SESSION_ID] [--worktree DIR] PROMPT)` — one-shot `claude -p` invocation with session persistence. Saves session ID to `SID_FILE`, reads it back on resume. `agent_recover_session()` — restore previous session ID from `SID_FILE` on startup. **Nudge guard**: skips nudge injection if the worktree is clean and no push is expected, preventing spurious re-invocations. Callers must define `SID_FILE`, `LOGFILE`, and `log()` before sourcing. **Concurrency**: every `claude` invocation is wrapped in `flock -w 600` on `${HOME}/.claude/session.lock` to serialize OAuth refresh across containers — see [`docs/CLAUDE-AUTH-CONCURRENCY.md`](../docs/CLAUDE-AUTH-CONCURRENCY.md) for why this is load-bearing and when a new container should bypass it. | formula-driven agents (dev-agent, planner-run, predictor-run, gardener-run) |
 | `lib/forge-setup.sh` | `setup_forge()` — Forgejo instance provisioning: creates admin user, bot accounts, org, repos (code + ops), configures webhooks, sets repo topics. Extracted from `bin/disinto`. Requires `FORGE_URL`, `FORGE_TOKEN`, `FACTORY_ROOT`. **Password storage (#361)**: after creating each bot account, stores its password in `.env` as `FORGE_<BOT>_PASS` (e.g. `FORGE_PASS`, `FORGE_REVIEW_PASS`, etc.) for use by `forge-push.sh`. | bin/disinto (init) |
 | `lib/forge-push.sh` | `push_to_forge()` — pushes a local clone to the Forgejo remote and verifies the push. `_assert_forge_push_globals()` validates required env vars before use. Requires `FORGE_URL`, `FORGE_PASS`, `FACTORY_ROOT`, `PRIMARY_BRANCH`. **Auth**: uses `FORGE_PASS` (bot password) for git HTTP push — Forgejo 11.x rejects API tokens for `git push` (#361). | bin/disinto (init) |
 | `lib/ops-setup.sh` | `setup_ops_repo()` — creates ops repo on Forgejo if it doesn't exist, configures bot collaborators, clones/initializes ops repo locally, seeds directory structure (vault, knowledge, evidence, sprints). Evidence subdirectories seeded: engagement/, red-team/, holdout/, evolution/, user-test/. Also seeds sprints/ for architect output. Exports `_ACTUAL_OPS_SLUG`. `migrate_ops_repo(ops_root, [primary_branch])` — idempotent migration helper that seeds missing directories and .gitkeep files on existing ops repos (pre-#407 deployments). | bin/disinto (init) |