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>

.env.example

@@ -7,6 +7,11 @@
 # Port to listen on (default: 4820)
 # DASHBOARD_PORT=4820
 
+# Directory the built client is served from in production (default: client/dist).
+# The `ccam` plugin install points this at ~/.claude/agent-dashboard/runtime/client-dist
+# because the plugin cache directory is read-only and replaced on every update.
+# DASHBOARD_CLIENT_DIST=
+
 # Interface to bind. SECURITY: defaults to 127.0.0.1 (loopback) so the dashboard
 # is NOT reachable from the network out of the box (GHSA-gr74-4xfh-6jw9). The
 # server reads transcripts, exports all data, and can spawn `claude`, so binding