Fix rules-to-skills migration (#58278)

Closes https://github.com/zed-industries/zed/issues/57992
Closes AI-349


## Summary
- Run rules-to-skills migration during agent UI init instead of waiting
for feature flags in production.
- Add a command palette action to rerun the migration manually.
- Make AGENTS.md migration appends idempotent to avoid duplicated
default-rule prompts.


Release Notes:

- Added a manual way to trigger the Rules to Skills migration