feat: implement Monitor tool for streaming shell output (#649)

* feat: implement Monitor tool for streaming shell output

Add the Monitor tool that executes shell commands in the background and
streams stdout line-by-line as notifications to the model. This enables
real-time monitoring of logs, builds, and long-running processes.

Implementation:
- MonitorTool (src/tools/MonitorTool/) — spawns LocalShellTask with
  kind='monitor', returns immediately with task ID
- MonitorMcpTask (src/tasks/MonitorMcpTask/) — task lifecycle management
  and agent cleanup via killMonitorMcpTasksForAgent()
- MonitorPermissionRequest — permission dialog component

The codebase already had all integration points wired (tools.ts, tasks.ts,
PermissionRequest.tsx, LocalShellTask kind='monitor', BashTool prompt).
This PR provides the missing implementations.

* fix: command-specific permission rule + architecture docs

- MonitorPermissionRequest: "don't ask again" now creates a
  command-prefix rule (like BashTool) instead of a blanket
  tool-name-only rule that would auto-allow all Monitor commands
- MonitorMcpTask: clarify architecture comments explaining why
  monitor_mcp type exists as a registry stub while actual tasks
  are local_bash with kind='monitor'

* fix: address Copilot review feedback

- Fix permission rule field: expression → ruleContent (Copilot #1)
- Handle empty command prefix: skip rule creation (Copilot #2)
- Remove unused useTheme() import (Copilot #3)
- Save permission rules under 'Bash' toolName so bashToolHasPermission
  can match them — Monitor delegates to Bash permission system (Copilot #4)
- Remove unused logError import from MonitorMcpTask (Copilot #6)
- Copilot #5 (getAppState throws): same pattern as BashTool:915, not a bug

scripts/build.ts

@@ -25,7 +25,7 @@ const featureFlags: Record<string, boolean> = {
   BRIDGE_MODE: false,
   DAEMON: false,
   AGENT_TRIGGERS: false,
-  MONITOR_TOOL: false,
+  MONITOR_TOOL: true,
   ABLATION_BASELINE: false,
   DUMP_SYSTEM_PROMPT: false,
   CACHED_MICROCOMPACT: false,