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.

Dockerfile.base

@@ -316,12 +316,18 @@ RUN if [ "${INSTALL_MEMPALACE}" = "true" ]; then \
 # kwarg alias so existing callers still work.
 #
 # Idempotent and self-deactivating: once upstream releases the fix the
-# regex no longer matches and this RUN is a silent no-op.
-# Upstream tracking:
+# regex no longer matches (and the WARN below fires) — that's the signal
+# to delete this RUN.
+# Upstream status (last checked 2026-06-14):
+#   issue #1728 — STILL OPEN (root-level anyOf rejected by Anthropic/Codex)
+#   PR #1735    — CLOSED UNMERGED 2026-06-11; do NOT watch it (dead)
+#   PR #1717    — open; the current live fix candidate to watch
+#   mempalace PyPI latest = 3.4.0 (== our pin) → no release contains the fix yet
 #   https://github.com/MemPalace/mempalace/issues/1728
-#   https://github.com/MemPalace/mempalace/pull/1735
-# TODO: remove this RUN once a mempalace release containing PR #1735 is on
-# PyPI and installed by the line above.
+#   https://github.com/MemPalace/mempalace/pull/1717
+# TODO: remove this RUN once a mempalace release > 3.4.0 that actually strips
+# the root-level anyOf ships on PyPI and is installed by the line above.
+# Keep MEMPALACE_VERSION in lockstep with opencode-devbox when bumping.
 RUN if [ "${INSTALL_MEMPALACE}" = "true" ]; then \
       MP_FILE="$(find /opt/uv-tools/mempalace -path '*/mempalace/mcp_server.py' | head -n1)" && \
       if [ -z "$MP_FILE" ]; then echo "mempalace mcp_server.py not found" >&2; exit 1; fi && \