feat(plugins): make CCAM installable straight from a Claude Code plugin

Adds a root `ccam` plugin (`.claude-plugin/plugin.json`, `"source": "./"`) so
`/plugin marketplace add` + `/plugin install ccam@...` is enough on a machine
with nothing but Claude Code: no clone, no npm run setup, no manual npm start.

- scripts/plugin-bootstrap.js: SessionStart hook. Fast-path exit, Node >=22.5
  gate (node:sqlite), mkdir lock with stale reclaim, deps installed into
  ~/.claude/agent-dashboard/runtime/ (never the plugin cache), legacy
  checkout-hook cleanup (backed up), ~/.local/bin/ccam launcher, eager UI
  build so client routes like /run work immediately, detached server spawn.
- scripts/plugin-open.js, scripts/plugin-doctor.js: /ccam-open, /ccam-doctor.
- server/index.js: DASHBOARD_CLIENT_DIST override (plugin cache is read-only).
- mcp/build/ is committed (plugin MCP servers start before any bootstrap could
  build them) and kept honest by scripts/check-mcp-build.js (content hash,
  not mtime), enforced by pre-commit when mcp/src changes.
- plugins/ccam-dashboard/.mcp.json moved under plugins/ccam/ with a working
  ${CLAUDE_PLUGIN_ROOT} path (the old relative path never resolved from a
  marketplace-cached subdir).
- Docs: README, INSTALL, SETUP, ARCHITECTURE, CLAUDE.md, docs/PLUGINS.md,
  docs/MCP.md, docs/CLI.md, docs/HOOKS.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/index.js

@@ -153,7 +153,13 @@ function startServer(app, port) {
 
   const isProduction = process.env.NODE_ENV === "production";
   if (isProduction) {
-    const clientDist = path.join(__dirname, "..", "client", "dist");
+    // The bundle normally sits in the checkout. A plugin install runs the
+    // server out of a read-only plugin cache directory, so DASHBOARD_CLIENT_DIST
+    // points it at the writable runtime dir instead (see
+    // scripts/plugin-bootstrap.js). A missing directory is harmless: the API
+    // serves fine and only the UI route 404s until the bundle is built.
+    const clientDist =
+      process.env.DASHBOARD_CLIENT_DIST || path.join(__dirname, "..", "client", "dist");
     // Cache policy designed to survive client rebuilds without forcing a hard
     // refresh:
     //   - Hashed bundles under /assets/ never change for a given URL, so cache