docs: CLAUDE.md — codify changelog-update rule

Add an explicit rule that every user-visible change updates
CHANGELOG.md in the same commit, under an `## [Unreleased]` heading
between releases. References the C-2 gap (0.2.7 and 0.2.8 shipped
without changelog entries) so the motivation is durable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -77,6 +77,14 @@ Only a second consecutive failure is treated as a real auth problem.
 - Type hints and docstrings everywhere
 - Formatter: `ruff format` · Linter: `ruff check` · Tests: `pytest`
 - All text (docstrings, comments, README): English
+- **CHANGELOG.md:** every user-visible change (bug fix, new/changed
+  tool, behavior change, security fix, dependency bump) gets a
+  `CHANGELOG.md` entry in the same commit — under a `## [Unreleased]`
+  heading between releases, which becomes `## [X.Y.Z] - YYYY-MM-DD` on
+  version bump. Pure internal cleanup (renames without external callers,
+  comment-only edits, ruff autofix) needs no entry. Don't ship a release
+  with a stale changelog (this was the C-2 gap that caused 0.2.7 and
+  0.2.8 to ship undocumented).
 - **Version consistency:** the package version lives in `pyproject.toml`
   and must stay in sync with `uv.lock` and the `[X.Y.Z]` heading in
   `CHANGELOG.md`. `src/mcp_synology_container/__init__.py` derives