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/tools/session_process_unix.go

@@ -0,0 +1,14 @@
+//go:build !windows
+
+package tools
+
+import (
+	"syscall"
+)
+
+func killProcessGroup(pid int) error {
+	if err := syscall.Kill(-pid, syscall.SIGKILL); err != nil {
+		_ = syscall.Kill(pid, syscall.SIGKILL)
+	}
+	return nil
+}