docs: adopt a changelog convention, and make Unreleased follow it

Brian asked what the standard is; there wasn't a written one, only a de facto
pattern in the history. This writes it down in CLAUDE.md and fixes the one
place the repo already diverged from it.

The rule: write the entry in the same commit as the work, under ## Unreleased;
cut the version on dev in a dedicated chore(release) commit; never touch the
changelog at a merge boundary. The entry goes in with the change because that
is the only moment you still know why.

Two additions beyond what the history already did:

No preamble under ## Unreleased. The themed opening paragraph gets written at
release time, when the whole release is visible and can be named honestly. The
current one proved the point — "Blastware Event/FFT-Report parity: the FFT,
the USBM compliance chart, and the sensor self-check" was accurate when the
first item landed and stopped being accurate once rescue-on-connect landed
under the same heading. Removed here; the release commit writes a new one
covering everything actually in the release.

And the operational consequence is now mandatory on any entry touching the
codec, the waveform store, or the DB — including when it is "none". This
repo's changelog is how future-you learns whether a deploy costs two hours on
the NAS, so silence is ambiguous and "none" is information. The old preamble's
load-bearing half is preserved as an explicit ### Migration block rather than
dropped with the prose around it.

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

CLAUDE.md

@@ -89,6 +89,36 @@ When new information about the protocol is discovered, please update the instant
 
 ---
 
+## Changelog & release convention
+
+**Write the entry in the same commit as the work, under `## Unreleased`.  Cut
+the version on `dev` in a dedicated release commit.  Never touch the changelog
+at a merge boundary.**
+
+- **Entry goes in with the change**, not at merge or release time — that is the
+  only moment you still know *why*.  Feature branches edit `CHANGELOG.md`
+  directly; the occasional conflict is two appended bullets and is trivial.
+- **No preamble under `## Unreleased`** — just the `### Added` / `### Changed` /
+  `### Fixed` lists.  The themed opening paragraph gets written at release
+  time, when the whole release is visible and can be named honestly.  A theme
+  written when the first item landed is stale by the third.
+- ⚠ **State the operational consequence** on any entry touching the codec, the
+  waveform store, or the DB — **including when it is "none."**  "requires
+  `backfill_sidecars.py` + `backfill_event_shape.py`, ~2 h on the NAS",
+  "`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.
+- **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
+  `README.md`, and `minimateplus/event_file_io.py` (`TOOL_VERSION`) **when the
+  codec changed** — that constant gates `.h5` regeneration.
+- **`main` carries only released versions.**  No `## Unreleased` section there;
+  it lands via the `dev` → `main` PR.  `main` lagging `dev` by a version is
+  normal.
+
+---
+
 ## Architecture: three-tier conceptual model
 
 seismo-relay is a **suite of cooperating components**, not a single app.