Scan only the current commit when release anchors are unusable

[RomainCscn]

When findBaseSha rejects every recent release (no commitSha recorded, or SHAs that aren't in this repository's history), the fallback scanned from the merge's first parent. On merge-based deploys that's the whole merged branch: hundreds of commits, re-discovering issues that shipped weeks ago and re-attaching them to the current release. In the reported case a single sync scanned 475 commits and re-added a batch of Done issues.

• Broken anchors (releases exist but none is usable) now scan only the current commit, with a warning that names the cause: SHAs missing from this repo's history (pipeline synced from multiple repositories) or releases carrying no SHA at all (created manually), plus the config fix for each.
• True first syncs (no releases yet) keep the first-parent boundary.
• Scans over 100 commits warn that already-shipped work in the range will be linked to the release.

The reduced sync still stamps the current commit on the release, so the next sync has a valid anchor and ranges go back to normal on their own.

Fixes LIN-78447

[linear-code]

LIN-78447