docs: correct mempalace anyOf workaround watch-target (PR #1735 is dead)

Parity with opencode-devbox: PR #1735 (the diary_write root-anyOf fix) was
closed UNMERGED on 2026-06-11, so the old "remove once PR #1735 ships" TODO
pointed at a dead PR. Issue #1728 is still open; PR #1717 is the current live
candidate; mempalace PyPI latest is still 3.4.0 (== our pin), so the
workaround stays.

- Dockerfile.base: rewrite the upstream-tracking comment + TODO (#1735 dead,
  watch #1717, removal trigger = a PyPI release > 3.4.0 stripping root anyOf).
- CHANGELOG: Unreleased Docs entry.

Docs-only; no behavior change.

CHANGELOG.md

@@ -37,6 +37,15 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
   — which hashes differently than a SHA and triggers one *extra* rebuild,
   never a *missed* one (fail-toward-rebuild).
 
+### Docs (no image change)
+
+- Correct the MemPalace `diary_write` anyOf workaround watch-target in
+  `Dockerfile.base`: upstream PR #1735 was **closed unmerged** (2026-06-11),
+  so the old “remove once #1735 ships” TODO pointed at a dead PR. Issue #1728
+  is still open; PR #1717 is the current live candidate; mempalace PyPI latest
+  is still 3.4.0 (== our pin), so the workaround stays. Removal trigger is now
+  a PyPI release > 3.4.0 that actually strips the root-level anyOf.
+
 ---
 
 ## v1.1.1 — 2026-06-13