fix: report actionable diagnostic for overlong unix socket paths

[Vrindakr3300]

Summary

Detects overlong Unix socket paths before bind, connect, or daemon spawn operations on Unix platforms, failing fast with an actionable diagnostic message instead of encountering a low-level bind failure or generic 15-second server readiness timeout.

Closes #3

Key Changes

• Path Validation & Diagnostic Error (src/ipc.rs):
  • Added max_unix_socket_path_len() inspecting libc::sockaddr_un's sun_path length minus null byte (103 bytes on macOS/BSDs, 107 bytes on Linux).
  • Added validate_socket_path() returning an InvalidInput error with the configured path, byte length vs. platform limit, and remediation suggestion (set a shorter NAGI_CONFIG_PATH or XDG_CONFIG_HOME).
  • Integrated path validation into bind_local_listener, connect_local_stream, and prepare_socket_path.
• Pre-spawn & Readiness Validation (src/server/autodetect.rs):
  • Validates both API (socket_path()) and client (client_socket_path()) paths at entry of auto_detect_launch() and wait_for_server_socket().
• Unit Coverage:
  • Added unit tests in src/ipc.rs, src/server/autodetect.rs, and src/server/socket_paths.rs verifying path limit enforcement and error diagnostics.

Example Output

socket path exceeds Unix domain socket limit (120 bytes > 103 bytes): /path/to/deeply/nested/nagi.sock; set a shorter NAGI_CONFIG_PATH or XDG_CONFIG_HOME