fix: collect selected checkout for sync and hooks

[flexdinesh]

• Behavior owner and resulting change: CLI sync and hook producers collect only the selected checkout; manual sync suppresses acknowledged unchanged uploads.
• Contracts/invariants affected; local/remote implications: preserve merge-base scopes, session associations, immutable retry IDs and destination/database pinning. New sessions reuse stored reviews; stale/deleted contexts recollect. Existing pending uploads remain recoverable.
• Validation results; remaining risks: full mise run check:push passed with isolated browser port 4186, including all 64 browser tests and Go race tests. Embedded assets clean.
• Intentional architecture/design changes and rationale: replace automatic worktree enumeration with checkout selection matching local review; update canonical docs and boundary tests.
• UI evidence: no frontend changes.