deps: pin curlewlabs-com/local-mutex to @v2.0.0

[jeffwall-curlewlabs]

Problem

Both action.yml and save/action.yml depended on curlewlabs-com/local-mutex@v1, a floating major tag that moves on every v1.x release. local-cache is itself a published action whose action.yml ships to public consumers, so a floating nested dependency means every local-mutex retag silently changes behavior for pinned local-cache@v3 consumers — defeating the point of pinning local-cache in the first place. A regression in a future local-mutex@v1 push would be inherited by every @v3 consumer with no announcement and no reviewable bump.

What Changed

Both uses: references now pin the exact immutable patch tag curlewlabs-com/local-mutex@v2.0.0 — the first release in local-mutex's v2 series. It adds the optional lock-dir input that we don't currently pass, but preserves every prior behavior under the default. The v2.0.0 tag exists in local-mutex's repo as an immutable patch tag (pushed once, never force-moved), so this pin is a true reproducibility anchor.

AGENTS.md gains a split-rule clarification:

• Internal monorepo CI workflows can still float their curlewlabs-com dependencies (we observe regressions in our own CI and react immediately).
• Dependencies nested inside a published action's action.yml must pin to an immutable tag because our public consumers can't react to our internal tag moves.

The rule spells out the exact pattern (@v2.0.0, not @v2), explains the reasoning, and notes that bumps to pick up new local-mutex features go through a dedicated PR.

Invariants / Risks

• No behavioral change for callers. local-mutex@v2.0.0 behaves identically to @v1.2.0 when lock-dir is not set; local-cache does not currently pass lock-dir, so all acquires continue to use the default /tmp path exactly as before.
• Existing concurrency tests in .github/workflows/ci.yml exercise the full restore/save flow against the pinned dependency — they're the behavioral regression guard.
• Future local-mutex updates no longer land automatically. Adopting a new release now requires a PR that bumps the pin — that's the intended cost of pinning.
• New diagnostic notices from local-mutex@v1.2.0 forward (waiting for lock … / released … at …) now appear in local-cache step logs. That's a mild UX improvement, not a regression.

Verification

• actionlint .github/workflows/ci.yml: clean.
• Full CI exercises restore and save against local-mutex@v2.0.0 — same concurrency invariants as before.

Follow-ups (separate PRs in this plan)

• SHA-256 of cache key for the mutex name (fixes long-key collision in the mutex namespace even though storage is already collision-safe).
• Reject dangerous restore targets before rm -rf.
• Update curlewlabs2/CLAUDE.md to record the split rule at the monorepo level.