feat: add mode-aware native mouse input

[pmarreck]

Add mode-aware mouse input for terminal automation through the native libghostty-vt backend. For example, a left press, move, and release now produces a drag using the child's negotiated tracking mode and wire format.

• Add the mouse CLI/RPC command and batch steps with zero-based row/column coordinates, modifiers, held-button state, wheel presses, and optional pixel cell dimensions.
• Pass native Buffers directly to the PTY and preserve exact bytes in input_mouse events as base64, including raw X10 bytes. Suppressed events return reported: false and still receive a sequence number.
• Expose the mouse-input capability and CAPABILITY_UNAVAILABLE (exit 12) for unsupported renderers or older native packages. Update schemas, replay/export handling, CLI docs, and public skill guidance together.

Depends on libghostty-vt-node PR #12. The optional dependency and lockfile retain the published package; they need updating after a mouse-capable native release. The development recipe pins tested binding revision 8318d03 and explains how to build/install its tarball. Native integration tests run when that API is available; AGENT_TTY_REQUIRE_MOUSE_NATIVE=1 makes missing support fail rather than skip.

Validation on x86_64 Linux with Node 24.18.1, npm 10.9.8, and binding 8318d03:

• AGENT_TTY_REQUIRE_MOUSE_NATIVE=1 npm run verify passes: formatting, lint, typecheck, all 1,673 tests across 161 files, build, and isolated tarball installation. Tested after merging upstream a269ca4 and installing its lockfile with aube 1.10.4 (Vitest 4.1.11). NixOS verification used steam-run, browser libraries, Python/Make/GCC, and an isolated home. npm 10.9.8 avoids the npm 12 pack JSON change in the existing packaging script.
• Real raw-mode PTY tests check SGR, SGR-pixels, X10, UTF-8, URXVT, disabled reporting, press/drag/release, concurrent held-button requests during backend startup, exact event-log bytes, and subsequent replay.
• Regression tests first demonstrated dropped held-button state during concurrent backend startup; holding the renderer lock through encoding, PTY delivery, and state commit fixes it.
• No screenshot rendering changes or architecture deviations.

GitHub Actions for head 47102f9 require maintainer approval (action_required). The PR is conflict-free; local verification does not imply upstream CI has run.