Expose repository and cache usage diagnostics

[flyingrobots]

Parent: #50
Related: #49

Outcome

Applications and operators can inspect git-cas usage without reimplementing Git object traversal or running destructive garbage collection.

Scope

• CacheSet entry count, logical bytes, object bytes where knowable, age, expiry, and policy.
• RootSet and Vault contribution summaries.
• Repository-wide anchored, orphaned-with-grace, and volatile classification from Repository-wide reachability classification for RootSet doctor #49.
• Explicit unknown or bounded states where Git cannot prove attribution.
• Machine-readable doctor output suitable for downstream composition.

Proof

• Synthetic repository containing branches, tags, reflogs, vault assets, root sets, recent unreachable objects, and immediately prunable objects.
• No command mutates or prunes repository state.
• Attribution limitations are represented rather than guessed.

[coderabbitai]

🔗 Related PRs

#18 - Deduplicate repeated chunk tree entries [merged]
#47 - Make GitHub issues the canonical tracker [merged]

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