docs: handoff-77, changelog-discipline rule, and #39 changelog entry

- docs/handoff/20260622-handoff-77.md — session handoff for the #39
  schema-cache-gate fix; records the four open issues (incl. the new #40).
- CLAUDE.md — add the changelog-discipline rule: update [Unreleased] in
  the same change as any user-facing behaviour change (scripted/pasted
  paths count), under the two copy rules; release-time sweep of
  commits/handoffs as the backstop.
- CHANGELOG.md — [Unreleased] → Fixed bullet for the #39 paste/scripted-
  input fix.

CLAUDE.md

@@ -246,6 +246,20 @@ Key invariants in the code:
   the specific product (SQLite, STRICT, rusqlite, PRAGMA).
   ADR-internal prose and code comments may name it where
   technically necessary for precision.
+- **Changelog discipline.** `CHANGELOG.md` (repo root) tracks
+  notable **user-facing** changes (Keep a Changelog + SemVer).
+  Update it in the **same change** that introduces a user-facing
+  behaviour change: add or amend a bullet under `[Unreleased]`
+  in the right category (Added / Changed / Deprecated / Removed /
+  Fixed / Security), phrased for end users under the two copy
+  rules above (no engine name; no "DSL" — say "simple mode" /
+  "advanced mode"). A change with no user-visible effect (pure
+  refactor, internal tests, CI plumbing) gets **no** entry — and
+  the judgement of "user-facing" includes scripted / pasted /
+  power-user paths, not just the interactive happy path. At
+  release time, rename `[Unreleased]` to the new version + date,
+  add the compare link, and **sweep the commits/handoffs since
+  the last tag** as a backstop for anything missed.
 - **Confirm commits.** Per the user's global rules, every
   `git commit` is preceded by an explicit message proposal
   and user approval. No AI attribution in commit messages.