## Summary

Adds an optional `filePath` parameter to `evaluate_script` that saves
the script output to a file instead of returning it inline.

Refs #153

## Motivation

Issue #153 requested `filePath` support for `take_snapshot` and
`evaluate_script`. `take_snapshot` was addressed in ChromeDevTools#463. PR ChromeDevTools#248
previously attempted this but was closed due to conflicts. This PR
implements the same feature on the current codebase, completing the
remaining piece.

## Changes

- Add optional `filePath` parameter to the `evaluate_script` schema
- Add `context.validatePath(filePath)` call for path validation
- Pass `{filePath, context}` options to `performEvaluation()`
- In `performEvaluation()`: when `filePath` is provided, save output via
`context.saveFile()` with `.json` extension; otherwise return inline as
before
- Update `docs/tool-reference.md` via `npm run docs:generate`
- Add unit test for file output

## Key design decisions

- **Same pattern as existing tools**: Follows the `context.saveFile()`
pattern established by `take_snapshot` (ChromeDevTools#463), `take_screenshot`,
`get_network_request` (ChromeDevTools#795), and performance tools (ChromeDevTools#686).
- **Minimal change surface**: Only `performEvaluation()` gains an
optional `options` parameter. No new interfaces or abstractions.
- **Backwards compatible**: `filePath` is optional. When omitted,
behavior is identical to before.

## Testing

**Unit test added** (`tests/tools/script.test.ts`):
- Call `evaluate_script` with `filePath` set to a temp file
- Assert response contains "Output saved to"
- Assert file content matches the JSON-serialized return value
- Clean up temp file in `finally` block

**Manual testing performed**:
- `() => document.title` with `filePath: /tmp/test.json` → file contains
`"Example Domain"`
- `() => document.title` without `filePath` → inline ```json block
returned (no regression)
- `() => Array.from({length: 100}, ...)` with `filePath` → 100-item
array saved correctly
- `filePath` pointing to non-existent directory → directory
auto-created, file saved
- Relative path (`test.json`) → resolved to CWD, absolute path shown in
response
- Function that throws → error returned, no partial file created
- Existing file as `filePath` → file overwritten completely

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Co-authored-by: Alex Rudenko <alexrudenko@chromium.org>