refactor: make all scripts multi-project via env vars

Replace hardcoded harb references across the entire codebase:
- HARB_REPO_ROOT → PROJECT_REPO_ROOT (with deprecated alias)
- Derive PROJECT_NAME from CODEBERG_REPO slug
- Add PRIMARY_BRANCH (master/main), WOODPECKER_REPO_ID env vars
- Parameterize worktree prefixes, docker container names, branch refs
- Genericize agent prompts (gardener, factory supervisor)
- Update best-practices docs to use $-vars, prefix harb lessons

All project-specific values now flow from .env → lib/env.sh → scripts.
Backward-compatible: existing harb setups work without .env changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev/dev-poll.sh

@@ -2,7 +2,7 @@
 # dev-poll.sh — Pull-based factory: find the next ready issue and start dev-agent
 #
 # Pull system: issues labeled "backlog" are candidates. An issue is READY when
-# ALL its dependency issues are closed AND their PRs are merged into master.
+# ALL its dependency issues are closed (and their PRs merged).
 # No "todo" label needed — readiness is derived from reality.
 #
 # Priority: