fix(git-sync): self-heal a missing 'main' branch that wedged a space (D3-N1)

Ref-store damage (a deleted refs/heads/main, an interrupted ref update) can leave an
existing vault repo without a 'main' branch. The cycle's ensureBranch('docmost','main')
+ checkout then throw every poll ("pathspec 'main' did not match"), wedging the space
forever with no self-heal — ensureRepo only creates branches on a FRESH git init
(found via web-test corruption charter, reproduced deterministically).

Add VaultGit.ensureMainBranch() and call it in the cycle preflight (after
clearStaleGitLocks, before the branch setup): if 'main' is missing, re-create it from
the 'docmost' mirror branch (they track each other) else from HEAD. Same
wedge-forever family as D3-N3.

Verified on the stand: deleting refs/heads/main now self-heals (main restored, the
edit reaches the vault, 0 pathspec errors) — was wedged forever. Unit test (real temp
repo: delete main -> ensureMainBranch restores it from docmost). git-sync suite green (708).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/git-sync/test/cycle.test.ts

@@ -17,6 +17,7 @@ function fakeVault(overrides: Record<string, any> = {}) {
     assertGitAvailable: rec("assertGitAvailable"),
     ensureRepo: rec("ensureRepo"),
     clearStaleGitLocks: rec("clearStaleGitLocks"),
+    ensureMainBranch: rec("ensureMainBranch"),
     isMergeInProgress: vi.fn(async () => false),
     ensureBranch: rec("ensureBranch"),
     checkout: rec("checkout"),