Auto-create private/ junctions in worktrees + browser extension template

[djdarcy]

Summary

Two related feature requests for RepoKit:

1. Auto-create junctions/symlinks for private/ in worktrees

When RepoKit creates worktrees (e.g., dev/, github/), the private/ directory only exists on the private branch. Other worktrees have no access to design docs, postmortems, or other private content.

Requested behavior: After creating worktrees, automatically create directory junctions (Windows) or symlinks (Unix) in each worktree's root so that dev/private/ and github/private/ point back to local/private/.

Why: Private docs (design docs, postmortems, Claude instructions) are useful from any worktree. Currently users must manually create junctions, which is tedious and easy to forget.

Implementation notes:

• Windows: mklink /J <target> <source> (junctions, no admin required)
• Unix/macOS: ln -s <source> <target>
• Should also add junction targets to .gitignore in each worktree to prevent accidental commits
• Consider making this configurable (opt-in or opt-out)

2. Browser extension project template (Chrome/Firefox)

RepoKit currently supports python, javascript, and generic language templates. There's no template for browser extensions, which have a distinct file structure.

Requested template: A browser extension template that generates:

• manifest.json (Manifest V3) with substitution variables
• content.js / background.js stubs
• popup.html / popup.js (optional)
• styles.css
• icons/ directory with placeholder sizes (16, 48, 128)
• Appropriate .gitignore entries for extension build artifacts

Could be implemented as:

• A new --language browser-extension option, OR
• A template category under languages/javascript/browser-extension/
• Should support both Chrome (Manifest V3) and Firefox (with browser_specific_settings)