v1.21.4 — merge from a linked worktree lands through a temporary one (#27)

[sshlg]

Fixes #27.

From a linked worktree, merge used to fail with could not check out main whenever another worktree held the integration branch. It now handles that case:

• Without --push: it refuses up front, touches nothing, and says why.
• With --push: the merge, the merge-log entry and the push happen in a temporary detached worktree, which is removed on every path. The holder's files and its ref are never touched.
• The holder's branch stays put. The fast-forward update-ref is skipped when another worktree has the branch checked out.
• Push before release. On both paths the push now happens first. If the push is rejected, the command fails and every lease stays held.

New check: check_merge_from_a_linked_worktree. Two mutations were caught:

• dropping the holder detection reproduces could not check out main;
• ignoring the push result is reported.

npm test:

• PASS: agent-sync v1.21.4 — all checks green
• SELF-TEST PASS (74 fixtures)
• claim cell 27, hooks 13, installer 11

Version 1.21.4 across the manifests, SKILL.md, the card, CHANGELOG and docs/evidence/verification.md.

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