[EXT] `registerBuiltinExtensions()` runs on every CLI invocation — consider lazy loading

[flyingrobots]

Problem

bin/git-mind.js calls registerBuiltinExtensions() at the top level on every invocation — including commands that don't need extensions at all (git mind init, git mind link, git mind process-commit, etc.).

This reads two YAML files from disk and validates them through AJV on every call. Currently fast enough to ignore, but worth flagging as the extension count grows.

Options

1. Lazy load: only call registerBuiltinExtensions() when an extension-aware command is running (view, extension list, etc.)
2. Keep eager but cache: if the files haven't changed (check mtime), skip revalidation
3. Do nothing: startup cost is negligible at 2 manifests

Label as tech-debt to revisit at scale.

Ref: #252

[flyingrobots]

Resolved in 45b29f2 (PR #270)