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>

plugins/ccam/commands/ccam-open.md

@@ -0,0 +1,29 @@
+---
+description: Build the dashboard UI if needed and print its URL
+---
+
+Build the dashboard bundle if it is not there yet, then print the URL. The
+bootstrap already builds it on session start, so this is usually a no-op — use
+it to force a rebuild, or to finish the build if the bootstrap's own attempt
+failed (check `~/.claude/agent-dashboard/runtime/client-build.log`).
+
+```bash
+node "${CLAUDE_PLUGIN_ROOT}/scripts/plugin-open.js"
+```
+
+The first run installs the client toolchain and takes a few minutes; later runs
+print the URL immediately. No server restart is needed — the server already
+serves from that directory.
+
+Then help the user open it:
+
+```bash
+uname -s
+```
+
+- `Darwin` → `open <url>`
+- `Linux` → `xdg-open <url>`
+- otherwise → tell them to open the URL in a browser.
+
+Keep the output to a few lines. Pass `--force` to the script only if the user
+asks for a rebuild.