feat(agent): add manage_bg_jobs tool to inspect and kill background bash jobs (#4577)

Detached bash jobs (#!bg) could be launched and auto-reported on completion,
but the agent had no way to act on a running one: no on-demand output read and
no kill (it blocked until the 1h max-runtime). bg_jobs had the pieces
(_read_output, list_for_session, internal _kill) but none was exposed.

Adds:
- bg_jobs.kill(job_id): tears down the process tree, marks the job killed, and
  sets followed_up so the monitor does not also auto-continue a deliberate kill.
- manage_bg_jobs registry tool with actions list / output / kill, scoped to the
  chat that launched the job (cross-session access reads as not found).
- Wiring: TOOL_HANDLERS/TAGS, function schema, RAG index + keyword hints, parser
  name map, dispatch (threads session_id via _direct_fallback). Gated like bash
  (NON_ADMIN_BLOCKED_TOOLS; plan-mode mutator).
- agent_loop: background-job intent regex maps to the files domain (and the tool
  joins _DOMAIN_TOOL_MAP[files]) so short commands like 'kill that job' are not
  dropped by the low-signal gate that skips tool retrieval.
- bg launch message tells the model to call manage_bg_jobs itself for check/stop
  rather than printing raw tool syntax to the user.

Tests: tests/test_bg_job_tools.py (kill semantics, per-chat scoping, actions,
and the intent classifier).

src/agent_tools/__init__.py

@@ -23,6 +23,7 @@ from .web_tools import WebSearchTool, WebFetchTool
 from .filesystem_tools import ReadFileTool, WriteFileTool, EditFileTool, LsTool, GlobTool, GrepTool, GetWorkspaceTool
 from .document_tools import CreateDocumentTool, UpdateDocumentTool, EditDocumentTool, SuggestDocumentTool, ManageDocumentTool
 from .model_interaction_tools import ChatWithModelTool, AskTeacherTool, ListModelsTool
+from .bg_job_tools import ManageBgJobsTool
 
 TOOL_HANDLERS = {
     "bash": BashTool().execute,
@@ -44,6 +45,7 @@ TOOL_HANDLERS = {
     "chat_with_model": ChatWithModelTool().execute,
     "ask_teacher": AskTeacherTool().execute,
     "list_models": ListModelsTool().execute,
+    "manage_bg_jobs": ManageBgJobsTool().execute,
 }
 
 # ---------------------------------------------------------------------------
@@ -56,7 +58,7 @@ PYTHON_TIMEOUT = 30
 
 # Tool types that trigger execution
 TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_file", "edit_file",
-             "grep", "glob", "ls", "get_workspace",
+             "grep", "glob", "ls", "get_workspace", "manage_bg_jobs",
              "create_document", "update_document", "edit_document",
              "search_chats",
              "chat_with_model", "create_session", "list_sessions",