Add built-in plugin system for namespaced target-scoped integrations

[burczu]

Summary

Closes #26.

This PR adds a built-in plugin system that lets target-scoped integrations (e.g. Rozenite) be added without leaking runtime-specific behavior into the core daemon, dispatcher, CLI, or protocol types.

• Protocol — one generic plugin-command IPC envelope added to IpcCommand; the union never needs to widen for a new plugin
• AgentPlugin interface (src/plugin.ts) — plugins declare a unique id, static commands, a supportsTarget() predicate, a state machine (idle / ready / unsupported-target / waiting-for-runtime / error), and optional daemon/target lifecycle hooks
• PluginOrchestrator (src/plugin-orchestrator.ts) — validates unique plugin/command ids at construction, wraps RuntimeSession into a narrow AgentPluginTargetSession, fans out lifecycle events, and dispatches plugin-command IPC with state enforcement before calling execute()
• AgentRuntimeBridge → plugin — migrated to src/plugins/runtime-bridge/index.ts implementing AgentPlugin; uses an injected CoreCommandRelay instead of holding the dispatcher directly; checks supportsTarget in lifecycle hooks (RN-only)
• Daemon — intercepts plugin-command before the dispatcher; sequences orchestrator lifecycle alongside core attachments via existing callbacks; calls orchestrator.start/stop around daemon lifetime
• CLI — createProgram gains a plugins: AgentPluginRegistration[] parameter; BUILT_IN_PLUGINS assembled in cli/index.ts; bridge registerCliCommands is a no-op placeholder for future subcommands

Backward compatibility

All existing commands, flags, outputs, and daemon behavior are unchanged. The plugin-command IPC type is additive. The runtime bridge behavior is identical — it now runs through the plugin seam transparently.

Risks

• The CoreCommandRelay is a closure over this.commandDispatcher constructed before the dispatcher is assigned. This is safe because the relay is only called at runtime, never during construction — same pattern as the original bridge.
• onTargetDisconnected is declared in the interface but not yet wired. No existing behavior changes.
• beforeClearTarget fires orchestrator cleanup as fire-and-forget (void). Plugin onTargetCleared implementations that do meaningful async work should be aware of this for the shutdown path.

Manual testing

Requires a React Native app with @agent-cdp/sdk installed (e.g. the playground app in this repo).

# build
pnpm build

# start daemon
pnpm run agent-cdp start

# confirm daemon is running and bridge plugin is active
pnpm run agent-cdp status

# select a React Native target
pnpm run agent-cdp target list
pnpm run agent-cdp target select <rn-target-id>

# trigger an in-app SDK measurement (via playground or your own app)
# verify it routes correctly — e.g. sample memory from the app
pnpm run agent-cdp memory usage list

# confirm unknown plugin-command returns a clean error
echo '{"type":"plugin-command","pluginId":"nope","command":"cmd"}' | nc -U ~/.agent-cdp/daemon.sock

# stop daemon
pnpm run agent-cdp stop

Expected: all existing workflows unchanged; unknown plugin command returns {"ok":false,"error":"Unknown plugin 'nope'"}.