Delegate WARP doctor, retention repair, and usage evidence to git-cas

[flyingrobots]

Parent: #734
Blocked by: #735 and git-stunts/git-cas#55

Outcome

git warp doctor explains WARP domain state while delegating physical CAS/cache inspection and repair to git-cas.

Scope

• Remove GitCasStateCacheRootSetCoordinator and WARP-owned retention report/repair implementations.
• Compose git-cas cache, RootSet, asset, age, size, expiry, and reachability evidence.
• Preserve WARP-specific coordinate and causal-publication explanations.
• Surface unrecoverable missing content honestly.
• Include retention witnesses in write/checkpoint/materialization receipts.

Proof

• Doctor reports live, stale, expired, malformed, missing, and collectible states.
• Repair calls git-cas operations and never recreates missing bytes.
• No destructive GC is run.
• State-cache payload refs are not anchored against Git GC #731 is closed or superseded only with source and reachability evidence.

[coderabbitai]

🔗 Related PRs

#64 - docs: enhance README with traversal directory, seek guide, and runtime compat [merged]
#93 - Fix recursive tree OID read fanout and release v17.0.1 [merged]
#107 - Prepare v18.0.0 release metadata and final gates [merged]
#708 - Fix live materialization state-cache hits [merged]
#710 - Release 18.2.1 [merged]

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