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/tool_schemas.py

@@ -1188,6 +1188,21 @@ FUNCTION_TOOL_SCHEMAS = [
             }
         }
     },
+    {
+        "type": "function",
+        "function": {
+            "name": "manage_bg_jobs",
+            "description": "Inspect and control detached background `bash` jobs (started with the `#!bg` marker). action='list' shows this chat's jobs with id/status/age/command; action='output' returns a job's captured output so far (use for a still-running job, or to re-read a finished one); action='kill' terminates a runaway job's process tree instead of waiting out its max-runtime. output and kill need job_id from list.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "action": {"type": "string", "enum": ["list", "output", "kill"], "description": "list | output | kill (default: list)"},
+                    "job_id": {"type": "string", "description": "Background job id (required for output/kill; from action='list')"},
+                },
+                "required": ["action"]
+            }
+        }
+    },
 ]