[EPAC-2009]: dedup CI failure Linear issues

[riddim-developer-bot]

Why

Flapping GitHub Actions workflows can create multiple urgent Linear issues for the same failing workflow and branch. This keeps one open ci-failure issue as the signal and records subsequent runs as capped comments.

What changed

• Replaced CI failure issue titles with deterministic [CI] {workflow_name} failing on {branch} titles.
• Added Linear exact-title dedup lookup scoped to EPAC, open state types, and the ci-failure label.
• Added capped duplicate-run comments and an HTML comment counter stored in the issue description.
• Added unit coverage for create, dedup-comment, comment-cap, duplicate-match warning, and GraphQL filter behavior.

Trade-offs not taken

• Did not add cross-run locking; concurrent failures can still race and create duplicates, which EPAC-2009 explicitly accepts.
• Did not implement success auto-close or project inheritance; those are out of scope for this issue.

Test plan

• python3 -m pytest scripts/ci/tests/test_ci_failure_to_linear.py -q
• python3 -m pytest scripts/ci/tests -q

Skipped external gate: release-manager manual validation still needs to inject 6 consecutive failures of the same workflow on main and verify the expected single issue/comment-cap behavior.

Reviewer-Boundary: review-only

Resolves EPAC-2009