feat(tools): add macOS computer-use built-in tools

[tokg-venv]

Summary

Adds a computer use tool set so the model can see the screen and act on it, as five new built-in tools (gated to darwin exactly like the speech tools):

Tool	What it does	macOS mechanism
computer_screenshot	Full-screen or region capture returned as image content plus a text note	screencapture (temp file read + always cleaned up)
computer_click	Left / right / double click at coordinates	CoreGraphics events posted via osascript JXA ObjC bridge (move settles hover first; double click sets ClickState 2)
computer_scroll	Scroll-wheel events in all four directions, 1-50 lines	CGEventCreateScrollWheelEvent
computer_type	Type into the focused control	System Events keystroke for ASCII; non-ASCII goes through a clipboard paste and the result discloses that
computer_key	Named special keys + modifier combos (cmd+shift+t, ctrl+up, ...)	key code / keystroke ... using {...} with a macOS key-code table

Zero new dependencies — everything shells out to tools macOS ships with. The image path uses the existing createToolCallResponse/ImageContent contract, so the screenshot is delivered to the model as a real image block.

Permission & safety notes

• macOS prompts for Screen Recording (screenshots) and Accessibility (clicks/typing/scroll) on first use; the tool descriptions tell the model about this.
• The tools are opt-in per thread like every built-in (the Add built-in tools dialog lists them automatically via the existing registry).
• The OS bridge (run/readFile/removeFile/temporaryPath) is injected, so the whole suite runs without touching the real screen, clipboard, or keyboard. The JXA CoreGraphics scripts and screencapture were verified live on macOS.

i18n boundary compliance (per AGENTS.md)

Tool names, descriptions, and execution-path result strings are model-facing and stay canonical English; no user-facing UI copy is added, so no message-tree keys are needed. Icons for the new tools are registered in the playground icon map.

Test plan

• New computer.test.ts (25 tests) driving every tool and error path through fake OS bridges: script construction for clicks/scrolls/keys, AppleScript escaping, ASCII vs clipboard typing, region validation, temp-file cleanup on failure
• Module registration order updated in built-in-tools-module.test.ts (darwin branch)
• Full bun run typecheck + ESLint clean; full bun test green except the two pre-existing langgraph-template failures that reproduce on clean main

[tokg-venv]

Self-reviewed the original submission against real hardware and fixed four real issues (05fe02a):

1. Retina coordinate drift (the big one) — screencapture returns 2x pixel images while clicks take points, and nothing told the model how to convert; on this machine a full-screen capture is 3420x2214 px over a 1710x1107 pt screen, so clicks aimed at image coordinates landed at 2x the intended spot. Screenshots now append a scale note to the text block ("Image: 3420x2214 px (2x points) … divide image pixel coordinates by 2"), derived by parsing the PNG IHDR and probing the main display via CoreGraphics (cached; region captures derive the scale from their own requested width so they degrade gracefully if the probe fails).
2. fn modifier always failed — System Events rejects using {fn down} at AppleScript compile time (verified via osacompile), so any fn+X combo errored; removed from the modifier table.
3. Fractional coordinates rejected — models dividing pixel coordinates naturally produce 855.5; the integer-only validation now accepts fractional points (negatives still rejected).
4. Screen probe returned null silently — the default screenPoints implementation called .trim() on execFileAsync's {stdout, stderr} object; the TypeError was swallowed by the fallback and every capture degraded to the no-scale note. Found by a live end-to-end run (the fake-backed unit tests can't catch it), fixed by destructuring, and the parser is now extracted for coverage.

All 21 computer tests pass (fakes now emit a real PNG header and a configurable screen size), full typecheck/lint green, and both capture modes were re-verified live on Retina.