fix(store): TaskAmended applies description/priority/due, not just title (S7)

Amendments to due/description/inherent_priority were accepted and durably
logged but silently discarded by the projection since store.apply only
ever handled the title key. Also added the missing Task.Description field
(TaskCreated never populated it either).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

progress.md

@@ -219,7 +219,14 @@ Fixed so far:
   `receipt` straight out of each `TaskCompleted` event's existing payload.
   Covered by an updated `TestBuildBrief`.
 
-Not yet started: Codex/opencode completion producers, S7–S11. See
+- **S7** — `TaskAmended` only ever applied `title` from the amendment
+  payload; `due`/`description`/`inherent_priority` amendments were accepted
+  and logged but silently dropped by the projection. Also added the missing
+  `Task.Description` field (it didn't exist at all, so `TaskCreated` dropped
+  it too). Both `TaskCreated` and `TaskAmended` now populate all four fields.
+  Covered by `TestTaskAmendedAppliesAllFields` (`internal/store`).
+
+Not yet started: Codex/opencode completion producers, S8–S11. See
 `AUDIT.md` for the full plan.
 
 **Phase 0 done (2026-07-27):** this box has live TCP reachability to the real