feat: three-state sync planner with warning semantics

- Plan is a pure function (inventory, manifest, disk facts, injected path
  mapping) emitting exactly six actions: deploy, update, skip+warn,
  leave+warn, remove, keep — deterministic order, no filesystem access
- full 3x3 matrix (disk unmodified/user-modified/never-managed x upstream
  added/changed/removed) table-tested plus missing-disk restore cases,
  a mixed fixture and a purity/determinism guarantee test
- executor applies plans item by item (atomic deploy/remove per item),
  rewrites the manifest once, appends timestamped SKIP/LEAVE lines with
  item name and reason to the sync log
- sync now plans before mutating: unmanaged occupants block new deploys,
  upstream deletions remove only unmodified copies, local modifications
  always win and are warned about

.beads/interactions.jsonl

@@ -16,3 +16,4 @@
 {"id":"int-c5834aa5e95b421826d076fd129d7a1a","kind":"field_change","created_at":"2026-08-22T19:54:32.168882535Z","actor":"m3ta-chiron","issue_id":"agent-lib-j2w.2","extra":{"field":"status","new_value":"closed","old_value":"in_progress"}}
 {"id":"int-b7efd31fd41bbb5acd082eb3c2f34dbd","kind":"field_change","created_at":"2026-08-22T19:56:21.734777976Z","actor":"m3ta-chiron","issue_id":"agent-lib-j2w.3","extra":{"field":"status","new_value":"closed","old_value":"in_progress"}}
 {"id":"int-d0f6b2e3946129f47ed8f071902a5c78","kind":"field_change","created_at":"2026-08-22T20:01:15.960626197Z","actor":"m3ta-chiron","issue_id":"agent-lib-j2w.4","extra":{"field":"status","new_value":"closed","old_value":"in_progress"}}
+{"id":"int-baf99558287967cfd7c3aeb9c0e6354c","kind":"field_change","created_at":"2026-08-23T08:31:08.602480836Z","actor":"m3ta-chiron","issue_id":"agent-lib-j2w.5","extra":{"field":"status","new_value":"closed","old_value":"in_progress"}}