fix(actions): cancel MV3 runs on draft transition

[seonghobae]

Summary

• subscribe the MV3 workflow to pull_request.converted_to_draft
• let that event enter the existing PR-scoped concurrency group and cancel an earlier running browser job
• keep the Draft job guard, so the cancellation event creates no replacement Chrome run

Traceability

• Successor to exact-head review finding: fix(actions): isolate MV3 PR runs #290 (comment)
• Stacked on fix(actions): isolate MV3 PR runs #290 exact ebeefcd534db4324498fdb18046ebc6255ddcdf2.

Test-first evidence

• RED: uv run pytest tests/test_mv3_compatibility_contract.py -q failed because converted_to_draft was absent from the trigger list.
• GREEN: same command reports 9 passed, 59 subtests passed; actionlint .github/workflows/mv3-compatibility.yml; git diff --check.

The event transition itself will be proven only by a hosted PR lifecycle run. No browser compatibility, release, deployment, or merge is claimed.

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[seonghobae]

Current-head governance/owner finding on 6a5c8e15aee9bcd89dc978e34f273727607d1227: this successor was opened Ready while its direct parent #290 is intentionally Draft because ebeefcd534db4324498fdb18046ebc6255ddcdf2 is based on stale #245 rather than the protected MV3 workflow generation. #290's live body also records that promoting its predecessor-relative workflow verbatim would regress protected converted_to_draft/closed lifecycle semantics and assigns reconstruction to canonical workflow owner #212. The new converted_to_draft cancellation intent is valid and should be preserved, but stacking another workflow mutation on that intentionally non-promotable parent does not make the stack Ready or establish protected-generation compatibility. I converted #306 back to Draft without changing head/base/source. Repair by carrying the valid cancellation intent into #212's authorized current-generation workflow reconstruction (preserving closed-event fail-closed behavior, concurrency/lifecycle, harden-runner/egress and immutable pins), then restack/retarget non-destructively and regenerate exact-head contracts plus hosted lifecycle evidence. Do not close merely to reduce queue count, and do not copy this stale workflow blob directly into protected main.