ai: support worktrees in claude code

[ovitrif]

This PR adds a .worktreeinclude file so Claude Code's claude -w worktree feature automatically copies build-critical gitignored files (signing keys, Firebase configs, local properties, Claude Code settings) into new worktrees.

Description

Defines gitignore-syntax patterns covering all gitignored files required for building and working with the project:

• local.properties (SDK path, GitHub credentials)
• keystore.* / *.keystore (signing configs and keystore binaries)
• google-services.json (Firebase, all flavors)
• *.local.* (Claude Code settings)
• tools/fcm-tester/service-account.json

This replaces the need to manually maintain multiple repo copies (bitkit-android-2, -3) for parallel work.

Preview

N/A — config file only, no UI changes.

QA Notes

1. Verify the file is tracked and patterns are correct
   • Run git ls-files --others --ignored --exclude-standard and confirm all critical files are listed
2. Test worktree creation
   • Run claude -w from the repo root
   • Verify the new worktree contains copies of local.properties, app/google-services.json, keystore.properties, .claude/settings.local.json
3. Verify worktree can build
   • In the new worktree, run ./gradlew compileDevDebugKotlin

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.