fix(release): make the branch-sync step content-derived, not per-component

[JSONbored]

Summary

The "Sync package-lock.json and engine-version pin" step in mcp-release-please.yml hardcoded for component in mcp engine miner ui-kit against the old release-please--branches--main--components--<name> naming. Since linked-versions (#7123) now puts mcp/engine/miner on ONE shared --groups--<groupName> branch, that hardcoded loop silently never found or synced it -- confirmed live: #7127's engine-and-dependents group branch left packages/loopover-miner/expected-engine.version stale, failing engine-parity:drift-check and two engine-version-display tests (manually patched directly on that branch to unblock it in the meantime).

Branches to check are now discovered via git ls-remote --heads against the release-please branch prefix instead of assumed by name, and the expected-engine.version sync fires on every checked-out branch (comparing its own committed pin against its own package.json) rather than an if component == engine check -- correct whether engine's bump lands via a solo branch or a shared group branch, and automatically adapts if the group's membership or name ever changes again.

Also deleted the 3 now-orphaned release-please--branches--main--components--{engine,mcp,miner} branches left over from before #7123 (all their PRs were already closed/merged; linked-versions will never reuse them).

Test plan

• npm run actionlint -- clean
• YAML-validated
• Verified the new git ls-remote --heads origin 'release-please--branches--main--*' glob matches exactly the real active release branch and nothing else (confirmed empty after deleting the 3 stale branches)
• Workflow-only change; not measured by Codecov. Real verification: chore(main): release engine-and-dependents libraries #7127 (now manually patched) should go green, and the next fresh release-please cycle should exercise this step end-to-end without manual intervention.

[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 95.98%. Comparing base (bf1c8b8) to head (bc547c7).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7128      +/-   ##
==========================================
+ Coverage   93.75%   95.98%   +2.22%     
==========================================
  Files         692      609      -83     
  Lines       68817    48154   -20663     
  Branches    18781    15133    -3648     
==========================================
- Hits        64520    46219   -18301     
+ Misses       3302     1118    -2184     
+ Partials      995      817     -178     

Flag	Coverage Δ
rees	?
shard-1	43.78% <ø> (-0.01%)	⬇️
shard-2	36.84% <ø> (-0.01%)	⬇️
shard-3	33.16% <ø> (+0.02%)	⬆️
shard-4	34.48% <ø> (-0.01%)	⬇️
shard-5	32.29% <ø> (-0.01%)	⬇️
shard-6	45.84% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 83 files with indirect coverage changes