Ship the batch: undo, parallel subagents, lazy MCP, hot-reloaded skills; docs and CI/CD

Loop and ergonomics batch across Now/Next and Maintenance:
- /undo and /redo via pre-prompt file snapshots (snapshot.ts)
- task takes a tasks[] array and runs investigations concurrently (subagent.ts)
- lazy MCP tools: mcp_list/mcp_inspect/mcp_call meta-tools, eager opt-in (mcp.ts, config.ts)
- skill tool reads its list live so a mid-session install is callable next turn (skills.ts)
- tool-name lists (tool-kinds.ts) derived from a mutating() marker; gates previously ungated writes
- prune/session recovery path summarized, and step-back doom-loop primitive (step-back.ts)
- @file completion re-walks on a slow cooldown; estimateTokens and pricing labeled as estimates

Docs: README, CHANGELOG, docs/{mcp,architecture,development} updated to match.
CI/CD: bun install-store cache and concurrency gates on both workflows; release.yml now
composes file-based release notes via scripts/make-release-notes.ts and verifies every binary.

scripts/make-release-notes.ts

@@ -0,0 +1,50 @@
+#!/usr/bin/env bun
+/**
+ * Writes `release_notes.md` for a release tag, extracting the tag's prose from
+ * CHANGELOG.md and appending the artifact inventory. Used by .github/workflows/
+ * release.yml so the release page reads like a hand-written one instead of the raw
+ * PR list.
+ *
+ * Reads the tag from $RELEASE_TAG (GITHUB_REF_NAME, e.g. `v1.2.3`) and fails loudly
+ * when CHANGELOG.md has no matching `## [<version>]` section, because a release with
+ * an empty body is worse than one that refused to publish.
+ */
+const tag = process.env['RELEASE_TAG'] ?? 'v0.0.0-local';
+const version = tag.replace(/^v/, '');
+
+const changelog = await Bun.file('CHANGELOG.md').text();
+const start = changelog.indexOf(`## [${version}]`);
+if (start === -1) {
+  console.error(`CHANGELOG.md has no "## [${version}]" heading for ${tag}`);
+  process.exit(1);
+}
+
+// The section runs from its heading to the next `## [` heading, or to the file end.
+// Drop the heading itself: the release title already carries the version, so keeping
+// `## [1.0.0]` under `## What's in v1.0.0` would read twice.
+let body = changelog.slice(start);
+const next = body.indexOf('\n## [', 1);
+if (next !== -1) body = body.slice(0, next);
+body = body.replace(/^## \[.*\]\n+/, '').trim();
+
+// ESM, so top-level await is fine; this file is only ever run as the entry script.
+await Bun.write(
+  'release_notes.md',
+  [
+    `## What's in ${tag}`,
+    '',
+    body,
+    '',
+    '### Artifacts',
+    '- `shiro-linux-x64`, `shiro-linux-arm64` - Linux',
+    '- `shiro-darwin-x64`, `shiro-darwin-arm64` - macOS',
+    '- `shiro-windows-x64.exe` - Windows',
+    '- `SHA256SUMS` - verify any of the above',
+    '',
+    'Install with `scripts/install.sh` (macOS/Linux) or `scripts/install.ps1` (Windows);',
+    'both verify the download against `SHA256SUMS`.',
+  ].join('\n'),
+);
+
+console.log(`composed release_notes.md from the ${version} changelog section (${body.split('\n').length} prose lines)`);
+process.exit(0);