Add VS Code settings for Wrap-up defaults and local override behavior

[jsmestad]

What

Expose Wrap-up controls in VS Code settings so users can set default behavior and local preferences that map to HeadsDown guidance cleanly.

Why

A default-on feature needs clear, inspectable controls to be trusted. Settings support personalization, reduce support load, and make opt-out behavior explicit.

Acceptance Criteria

1. Settings UI includes Wrap-up default mode with options auto, wrap_up, and full_depth.
2. Settings UI includes Wrap-up threshold minutes with validation range and help text.
3. Settings values are resolved using the existing precedence chain (VS Code settings, shared config fallback, defaults).
4. The extension uses current settings values when generating wrap-up-related quick actions and UI labels.
5. Documentation is updated in README.md with examples and expected behavior.

Developer Notes

Approach: Extend SettingsManager and package.json contributions to include wrap-up settings. Keep defaults aligned with product direction (auto, 30 minutes).

Steps:

1. Add configuration schema entries for wrap-up settings.
   • Files: package.json
2. Extend HeadsDownSettings, defaults, and resolution logic.
   • Files: src/settings.ts
3. Wire settings into wrap-up UI/quick-action behavior.
   • Files: src/extension.ts, src/status-bar.ts
4. Update extension documentation.
   • Files: README.md
5. Add tests for settings resolution and validation behavior.
   • Files: test/* (settings related tests)

Testing:

• Verify settings precedence behavior.
• Verify invalid values fall back safely.
• Verify README examples match actual behavior.

[jsmestad]

Dependency note: this ticket depends on the shared Finish Today (Wrap-up Guidance) API/SDK contract in https://github.com/headsdownapp/heads_down/issues/812. Implementation should start after that contract lands (or behind feature-flagged SDK prerelease).