docs: keep a CHANGELOG.md, with one entry file per PR

Release notes were written by hand on GitHub at tag time, so the contents
of a stacked release had to be pieced together from every PR, and output
changes that scripts and agents rely on were easy to miss.

- CHANGELOG.md backfills every published release (0.2.0 to 0.4.0) from its
  GitHub notes, verbatim.
- A PR adds changes/unreleased/<branch-name>.md instead of editing
  CHANGELOG.md, so stacked PRs never conflict. changes/README.md gives the
  sections, including Output Changes for JSON, MCP and error-code changes.
- scripts/changelog.sh: `check` validates entries and CHANGELOG.md,
  `release X.Y.Z` folds the entries into a new section (ordered by PR), and
  `notes X.Y.Z` prints a section as GitHub release notes.
- The release workflow sets the GitHub notes from the tag's section.
- New flake check `changelog` runs scripts/changelog-test.sh; both scripts
  are shellchecked.
- Entries for 0.5.0: #73, #74, #75, #78, #79, #80, #81 and this PR.

Claude-Session: https://claude.ai/code/session_01RqPPVeVDrnLoP7NUZmLRxY

flake.nix

@@ -152,6 +152,15 @@
           INSTALL_SH_SHELL=${shell} bash scripts/install-sh-test.sh
           touch "$out"
         '';
+        changelogSrc = lib.fileset.toSource {
+          root = ./.;
+          fileset = lib.fileset.unions [
+            ./CHANGELOG.md
+            ./changes
+            ./scripts/changelog.sh
+            ./scripts/changelog-test.sh
+          ];
+        };
         publisherTestSrc = lib.fileset.toSource {
           root = ./.;
           fileset = lib.fileset.unions [
@@ -303,6 +312,17 @@
               diff -u ${./web/src/content/docs/mcp/tools.md} tools.md
               touch "$out"
             '';
+          # CHANGELOG.md and changes/unreleased/ entries are well formed, and
+          # scripts/changelog.sh folds and extracts them as releases expect.
+          changelog = pkgs.runCommand "idx-changelog" {
+            nativeBuildInputs = with pkgs; [ bash coreutils diffutils findutils gawk gnugrep ];
+          } ''
+            cp -r ${changelogSrc}/. .
+            chmod -R u+w .
+            export HOME="$TMPDIR"
+            bash scripts/changelog-test.sh
+            touch "$out"
+          '';
           install-script-sh = installerCheck "sh";
           install-script-dash = installerCheck "dash";
           shellcheck = pkgs.runCommand "idx-shellcheck" {
@@ -311,6 +331,7 @@
             shellcheck -s sh ${installerSrc}/install.sh
             shellcheck ${installerSrc}/scripts/install-sh-test.sh
             shellcheck ${./scripts/live-smoke.sh}
+            shellcheck ${changelogSrc}/scripts/changelog.sh ${changelogSrc}/scripts/changelog-test.sh
             shellcheck ${./tests/agents/agent-smoke.sh}
             shellcheck ${./web/scripts/gen-cli-reference.sh} ${./web/scripts/gen-mcp-tools.sh}
             touch "$out"