[Misc] Compare a branch's plugin version against its merge base, not master's tip

[vmassol]

Jira URL

None — [Misc], follow-up to #99.

Changes

Description

Invariant 6 (added in #99) compared the branch's five version fields against origin/master's
tip. That answers the wrong question, now that master keeps releasing while a branch is open: a
branch that forked when the version was 1.1.7 and never touched a version field still differs
from a master that has since reached 1.5.1, so it would be told to revert a bump it never made.

Compare against the merge base instead — the point where the branch forked — which is what "did
this branch change the version?" actually means. The failure message now also names the merge base
and the version it expects, since on a stale branch that value is not obvious.

Clarifications

Found while stripping the version bumps out of the five open PRs (#18, #77, #81, #94, #98), whose
fork points range from 1.1.7 to 1.5.0 against a master now at 1.5.1. Without this fix, every
one of them would have gone red the moment its bump was removed — the exact opposite of the intent.

scripts/release.mjs is unaffected: it measures from the last release tag, not from a branch.

Screenshots & Video

Verified against the real PR #18 branch (fork point 986ed9f, version 1.1.7) with master at
1.5.1. As the PR stands today, having bumped the version:

validate.mjs: 1 consistency violation(s):
  - This branch changes the plugin version (... : 1.1.7 -> 1.1.8), but a pull request must not - it
    makes every concurrent PR conflict. Restore the version fields to the 1.1.7 they have at the
    merge base with master (986ed9fd); the release is cut on master by scripts/release.mjs. ...

And with the bump removed, i.e. the version left at its fork-point value of 1.1.7 while master says
1.5.1 — this is the case that was previously a false failure:

validate.mjs: OK (22 skills, Claude + Kimi + opencode versions in sync at 1.1.7, OKF map complete)

Executed Tests

node scripts/validate.mjs        # OK on this branch
claude plugin validate ./xwiki   # Validation passed

Plus the two cases above, run against the real pull/18/head in a throwaway clone with origin/master
pointed at the current master, so nothing was pushed anywhere.

Nothing under xwiki/ changes here, so this correctly triggers no release.

Expected merging strategy

Prefers squash: Yes. No backport branches.

🤖 Generated with Claude Code