Fix .git/config lock race when creating worktrees concurrently

[selfcontained]

Problem

git worktree add -b <branch> <path> origin/main auto-writes branch tracking config (branch.<name>.{remote,merge}) into .git/config, which takes the repo-wide config lock. When two worktree agents are created against the same repo at the same instant, the second worktree add loses the lock race:

error: could not lock config file .git/config: File exists
error: unable to write upstream branch configuration

git treats this as fatal (exit 255) and fails the whole command, so POST /api/v1/agents returns a 500. Surfaced as an intermittent E2E failure in worktree.spec.ts (whose setup creates several worktree agents in parallel) that aborted the E2E run. Root-caused from the preserved Playwright trace (500 body contained the config-lock error).

Fix

Pass --no-track to git worktree add so it no longer writes tracking config. That removes the only .git/config write from the non-idempotent command, so it can't lose the race. Upstream is still set explicitly right after via git branch --set-upstream-to (unchanged), which already tolerates exit 1/128 — git returns exit 1 when that write hits a contended lock, and a missing upstream degrades correctly to origin/<base> in worktree-status.ts and base-ref.ts, so creation succeeds either way. The end-state upstream (origin/main) is identical to git's auto-track output.

Tests

• worktree add carries --no-track and sets upstream explicitly.
• Creation succeeds when the upstream write is blocked by a contended config lock (exit 1 + lock stderr) — matches real git behavior (verified against git 2.50.1: exit 1, not 255).
• Updated the two existing worktree add command assertions for the new --no-track flag.

Validation

• pnpm run check ✓
• pnpm run test ✓ (server 2281 pass / 8 skip, +2 net new tests)
• pnpm run test:e2e ✓ (168 passed / 13 skipped; previously-failing worktree.spec.ts:480 now green)

A review agent flagged that an earlier draft's retry loop was dead code (git returns exit 1, which allowedExitCodes swallows before any throw); this PR drops the retry in favor of the deterministic --no-track fix per that feedback.

🤖 Generated with Claude Code