Remove exec agent — replaced by OpenClaw skill + vault API (#722)

## What

Removes the exec agent (PR #697). Its functionality is replaced by:

1. **OpenClaw skill** — teaches any OpenClaw instance to be the factory's face
2. **Vault API** — structured interface for proposals, approvals, rejections

The exec agent was rebuilding OpenClaw in bash. Every piece has a native OpenClaw equivalent:
- CHARACTER.md → SOUL.md
- exec/MEMORY.md → MEMORY.md
- exec-session.sh → session management
- exec-briefing.sh → heartbeats/cron
- Matrix dispatch → channel plugins

## Why

Prudence isn't a separate agent. She's what OpenClaw becomes when it has the disinto skill. One LLM, one vault API, no LLM-to-LLM.

## Related

- #721 — remove escalation, route through vault
- #709 — skill registry research
- #466 — example project (vault should have handled this, not escalation)

Co-authored-by: openhands <openhands@all-hands.dev>
Reviewed-on: https://codeberg.org/johba/disinto/pulls/722

.env.example

@@ -55,8 +55,3 @@ BASE_RPC_URL=                              # [SECRET] on-chain RPC endpoint
 # ── Tuning ────────────────────────────────────────────────────────────────
 CLAUDE_TIMEOUT=7200                        # [CONFIG] max seconds per Claude invocation
 
-# ── Executive Assistant ──────────────────────────────────────────────────
-# The compass is the exec agent's core identity — it lives outside the repo
-# so the factory cannot modify it. `disinto init` downloads it automatically
-# from disinto.ai/compass.md to ~/.disinto/compass.md.
-EXEC_COMPASS=                              # [CONFIG] path to compass file (default: ~/.disinto/compass.md)