feat(tools): add exec tool enhancement with background execution and PTY support (#1752)

- Unified exec tool with actions: run/list/poll/read/write/send-keys/kill
- PTY support using creack/pty library
- Process session management with background execution
- Process group kill for cleaning up child processes
- Session cleanup: 30-minute TTL for old sessions
- Output buffer: 100MB limit with truncation

Actions:
- run: execute command (sync or background)
- list: list all sessions
- poll: check session status
- read: read session output
- write: send input to session stdin
- send-keys: send special keys (up, down, ctrl-c, enter, etc.)
- kill: terminate session

Tests:
- PTY: allowed commands, write/read, poll, kill, process group kill
- Non-PTY: background execution, list, read, write, poll, kill, process group kill
- Session management: add/get/remove/list/cleanup

pkg/agent/instance_test.go

@@ -236,8 +236,9 @@ func TestNewAgentInstance_AllowsMediaTempDirForReadListAndExec(t *testing.T) {
 		t.Fatal("exec tool not registered")
 	}
 	execResult := execTool.Execute(context.Background(), map[string]any{
-		"command":     "cat " + filepath.Base(mediaPath),
-		"working_dir": mediaDir,
+		"action":  "run",
+		"command": "cat " + filepath.Base(mediaPath),
+		"cwd":     mediaDir,
 	})
 	if execResult.IsError {
 		t.Fatalf("exec should allow media temp dir, got: %s", execResult.ForLLM)