docs: make the release cadence explicit

Brian described the practice: Unreleased is the staging area for what is going
into the next release, and the version bump happens when enough has
accumulated to be worth shipping — not per commit, not per merge. The
convention already implied it ("never touch the changelog at a merge
boundary") but never said it outright.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qcu9ByJfuKBQxmrWb8rSrN

CLAUDE.md

@@ -108,6 +108,10 @@ at a merge boundary.**
   "`TOOL_VERSION` bumped", "no schema change, no migration".  Silence is
   ambiguous; "none" is information.  This repo's changelog is how future-you
   learns whether a deploy costs two hours.
+- **Releases are cut on judgement, not on a schedule or a merge.**  `Unreleased`
+  is the staging area for whatever is going into the next release; when enough
+  has accumulated to be worth shipping, it gets a number and a date.  Nothing
+  about a merge to `dev` triggers a release.
 - **Cutting a release** is its own `chore(release): vX.Y.Z — <theme>` commit on
   `dev`, renaming `## Unreleased` → `## vX.Y.Z — YYYY-MM-DD` and touching:
   `CHANGELOG.md`, `pyproject.toml`, the version line in `CLAUDE.md` and