feat(#275): Compound phase merged-PR guard and docs-only diff validation#

Problem#

The compound phase was running before the task's PR was merged, and could produce artefacts that included src/ or test/ files — both are incorrect behaviors that needed runtime enforcement.

Task / Link#

Closes #275

Changes#

• GitHubService.isPrMerged — new method that returns true when a PR is merged, false on 404 or open PR
• PhaseRouterService pre-compound guard — blocks compound with BLOCKED:PERSISTENT if task.prNumber is null; BLOCKED:TRANSIENT if PR is not yet merged
• PhaseRouterService post-compound diff validation — after compound COMPLETE, checks git diff in REPO_ROOT for src/ or test/ files; retries once with COMPOUND_DOCS_ONLY=1; signals BLOCKED:PERSISTENT on second violation
• InternalAdapterService.advanceAndEnqueue — fail-fast guard that blocks compound enqueue when task.prNumber is null (complementary to the runtime check above)

Notes#

• Diff check fails open (returns false) when REPO_ROOT is unset or git diff errors — pipeline continues rather than false-blocking
• COMPOUND_DOCS_ONLY env var is passed to the compound skill on retry; the skill must respect it to avoid another violation
• The InternalAdapterService guard is a fail-fast optimization; the PhaseRouterService guard is the authoritative check

Testing#

• Unit tests added for all acceptance criteria: isPrMerged (merged/open/404), compound guard (null prNumber, unmerged PR, merged PR), diff validation (clean, first violation retry, second violation block), and advanceAndEnqueue compound guard (null prNumber blocked, prNumber set proceeds)
• npm run test: 705/705 passing
• npm run lint: 0 errors