refactor(api): limit project mirror to Target Date only

apps/api/README.md

@@ -16,9 +16,13 @@ Live at `https://srs-api.prdlk.workers.dev`.
   never overwrites SRS-owned columns (`stage`, `next_review`).
 - **The repo owns the schedule** — `data/schedule.json`, human-edited,
   bundled at deploy. Git history is its audit log.
-- The GitHub Project mirror (`Target Date`, `SRS Stage`, `First Attempt`)
-  is best-effort: failures log warnings, never block a D1 write. Topic rows
-  are never written.
+- The GitHub Project mirror writes `Target Date` and nothing else: it is
+  best-effort, so failures log warnings and never block a D1 write, and topic
+  rows are never written. SRS stage and first-attempt result stay in D1 only —
+  the charts, digest and `/api/stats` read them from there, so mirroring them
+  into Project single-selects bought only drift. Do not re-add them.
+  `Set`/`Difficulty` are a projection of the issue labels, reconciled by
+  `bun run sync-project-fields` at the repo root, not by this Worker.
 
 ## The ladder