Polish changelog fragments with claude -p at release time

- Replace `renderGroupedChanges` with `polishFragmentsWithClaude` that pipes fragments through `claude -p --model sonnet` to merge related items, drop housekeeping noise, and produce user-facing release notes
- Internal fragments kept in CHANGELOG.md under a `<details>` collapse; dropped from GitHub release notes entirely
- CI no longer auto-runs `changelog:build` on tag-based releases — fails fast with a clear error if `changes/*.md` fragments are still pending; build locally and commit before tagging
- Add `runClaude` dep-injection seam to test surface; add failure-mode coverage (missing binary, empty output, missing headers, missing `<details>` wrapper)
- Delete implemented design doc; update `changes/README.md` and `docs/RELEASING.md` with claude CLI prerequisite and new workflow

src/renderer/modals/session-help.test.ts

@@ -2,10 +2,7 @@ import assert from 'node:assert/strict';
 import test from 'node:test';
 
 import { SPECIAL_COMMANDS } from '../../config/definitions';
-import {
-  describeSessionHelpCommand,
-  formatSessionHelpKeybinding,
-} from './session-help.js';
+import { describeSessionHelpCommand, formatSessionHelpKeybinding } from './session-help.js';
 
 test('session help describes sub-seek commands as subtitle-line navigation', () => {
   assert.equal(describeSessionHelpCommand(['sub-seek', 1]), 'Jump to next subtitle');