fix(review): replace the live in-flight dispatch check with a persisted R2 marker

[JSONbored]

Summary

• Follow-up to feat(review): GitHub-Actions build-and-serve visual-capture fallback #4131 (merged): the gittensory review on that PR flagged that hasInFlightFallbackDispatch's live GitHub runs-API query has a real eventual-consistency gap — a freshly-dispatched run isn't guaranteed to be visible via that API the instant dispatch returns, so a recapture-poll landing in that narrow window could still redispatch and get itself cancelled by the workflow's own concurrency: cancel-in-progress: true group.
• Replaces the live query with a persisted R2 sentinel (isFallbackDispatchInFlight/markFallbackDispatched/clearFallbackDispatchMarker), written synchronously right after a successful dispatch (no API round-trip to race against), cleared by the workflow_run webhook handler on ANY conclusion (success/failure/cancelled/timed_out — not just success, so a genuinely failed run doesn't block a retry for the rest of the window), and self-expiring after the workflow's 15-minute timeout-minutes (+ a queueing buffer) as a fail-safe if a webhook delivery is ever lost.
• Both write helpers use a real try/catch rather than .catch() on the R2 call's return value — a minimal/partial R2Bucket implementation that doesn't implement a given method throws synchronously at the call site, before .catch() would even attach. Caught this via a test-mock gap while writing regression tests, not from a production report.

Scope

• Conventional Commit title.
• Focused — one fix (the dispatch-marker mechanism), no unrelated changes.
• Follows CONTRIBUTING.md.
• Advances feat(review): GitHub-Actions build-and-serve fallback for repos with no CI preview deploy #4112 (the parent feature issue; already substantially shipped via feat(review): GitHub-Actions build-and-serve visual-capture fallback #4131).

Validation

• git diff --check
• npm run typecheck
• Targeted tests: test/unit/actions-fallback.test.ts, test/unit/actions-fallback-webhook.test.ts, test/unit/visual-capture.test.ts, test/unit/visual-config-wiring.test.ts, test/unit/focus-manifest.test.ts, test/unit/queue.test.ts — all 1411 tests pass.
• Scoped coverage on the two changed source files: actions-fallback.ts 100% stmts/branch/funcs/lines; capture.ts's one uncovered line (236) is pre-existing, unrelated code this PR didn't touch.
• npm run test:workers, build:mcp, test:mcp-pack, ui:openapi:check, ui:lint, ui:typecheck, ui:build, npm audit — not re-run locally for this exact commit (none of these files touch the OpenAPI/MCP/UI surfaces); CI runs them authoritatively.

Safety

• No secrets, wallet/hotkey/trust-score/reward data anywhere.
• No auth/cookie/CORS/session changes.
• No API/OpenAPI/MCP surface touched.
• No UI changes (backend only) — no UI Evidence section needed.
• No docs/changelog changes needed.

Notes

• Default-off feature (review.visual.actions_fallback), unaffected by this change either way — this only hardens the dispatch guard's own internal correctness.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (ca3f8ab) to head (2a93f21).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4133   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files         385      385           
  Lines       36195    36207   +12     
  Branches    13263    13265    +2     
=======================================
+ Hits        33919    33932   +13     
  Misses       1618     1618           
+ Partials      658      657    -1     

Files with missing lines	Coverage Δ
src/queue/processors.ts	95.18% <100.00%> (+<0.01%)	⬆️
src/review/visual/actions-fallback.ts	100.00% <100.00%> (+0.69%)	⬆️
src/review/visual/capture.ts	91.03% <100.00%> (+0.18%)	⬆️

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