Recover task file-context metadata backups before returning an empty context #2177

The JSON persistence caller audit identified a metadata read-recovery gap.

Source: src/core/context-tracking/FileContextTracker.ts.

saveTaskMetadata uses safeWriteJson. getTaskMetadata checks the canonical file and returns an empty files_in_context list if that file is absent. It does not recover an orphaned committed backup. A later save can replace recovered metadata with an empty or incomplete snapshot.

Proposed scope:

• Use the shared read-recovery protocol before selecting the empty default.
• Preserve the legitimate missing-metadata behavior.
• Do not treat failed backup recovery as evidence that no metadata exists.
• Keep failures observable without discarding committed metadata.

Acceptance:

• A metadata read restores the committed file-context list from an orphaned backup.
• A genuinely absent file returns the existing empty default.
• A failed restore preserves the backup.
• Unavailable reads cannot cause a later save to discard committed entries silently.

Evidence: source inspection. Add focused FileContextTracker tests.