Default localCacheMode to move-directory; add EXDEV copy fallback

[frostebite]

Summary

Stacked on #40 (canonical-overlay removal) — second piece of the game-ci/roadmap#11 caching workstream.

• Switches the default localCacheMode from tar to move-directory. tar archives + extracts (copies bytes twice) on every restore/save; move-directory is an O(1) atomic rename. This matches the pattern child-workspace-service.ts already uses successfully.
• tar remains fully available as an explicit opt-in (localCacheMode: tar) — still the right choice when distributing cache entries via the existing rclone/S3 built-in container hooks, since those push/pull tar archives.
• Adds an EXDEV/EPERM → copy fallback (renameOrCopy helper) in both local-cache-service.ts and child-workspace-service.ts. Previously a cross-volume rename (e.g. localCacheRoot/parentCacheRoot misconfigured on a different volume than the workspace) was an unhandled failure — it threw, and the caller's catch-all just discarded the cache and started fresh. Now it degrades gracefully to a copy + source cleanup (same net effect as a successful rename, just slower).

Why

Audit and rationale in game-ci/roadmap#11 (workstream 1). Short version: tar's double-copy cost is the most expensive path for large Library trees and shouldn't be the silent default; move-directory already existed and already worked, it just wasn't chosen automatically. The EXDEV gap was a real correctness issue independent of the default question — worth fixing regardless of which mode is default.

Test plan

• yarn typecheck — clean
• yarn lint — 0 errors
• yarn test — 923/924 passing (1 pre-existing, unrelated flaky timeout in cli-integration.test.ts, confirmed passing in isolation and untouched by this diff)
• Updated 2 existing tests that implicitly relied on tar being the default to explicitly request tar mode
• Added 4 new tests: default-mode rename on both restore and save, EXDEV fallback on both restore and save

Ref: game-ci/roadmap#11

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[frostebite]

⚠️ This PR shows as Merged, but its content never actually reached main. It was stacked on #40's branch and merged into that branch — but #40's branch was itself squash-merged into main 12 seconds before this merge happened, so this PR's payload (default cacheMode → move-directory, EXDEV/EPERM fallback, cacheMode rename) landed on a now-orphaned branch instead.

Superseded by #44, which cherry-picks this PR's actual content onto current main (plus one more fix — the deprecation warning was firing on every read instead of once).