[EXT] Extension registration is ephemeral — not persisted across invocations

[flyingrobots]

Problem

git mind extension add <path> registers the extension for the current process only. On the next invocation, the extension is gone.

Expected

Registered extensions should persist. The manifest path should be stored somewhere (e.g., .git/git-mind/extensions.yaml or .git-mind/extensions.yaml in the repo root) and reloaded at startup alongside the built-ins.

Impact

Makes extension add effectively useless for anything other than one-shot scripts until this is fixed.

Approach

• On extension add: append the manifest path to a config file
• At startup (bin/git-mind.js): after registerBuiltinExtensions(), load any user-registered extensions from the config file
• On extension remove: remove the path from the config file

Ref: #252