Add a clerk-migrate skill for moving users into Clerk

[royanger]

What does this solve? What changed?

Adds clerk-migrate, a skill for moving users into Clerk with the CLI's clerk migrate commands: export from the old provider, check and import, then inspect or undo the run. It documents the commands from the clerk/cli#479 stack as they stand in its last PR, clerk/cli#531.

Files in skills/clerk-migrate/:

• SKILL.md: routing, targeting, the five-step import flow (identify the source, summarize, dry run, consent, report), and a summary of re-runs and undo
• references/export.md: exporting from Auth0, Supabase, Firebase, Auth.js, Better Auth, WorkOS and Clerk
• references/sources.md: what each built-in source carries, and how to write one for any other platform
• references/checks.md: each reject reason in the order the checks apply them, the warnings, --allow-partial, and exit codes
• references/runs.md: re-running, run files and statuses, and every undo case
• references/clerk-to-clerk.md: development to production

The clerk router (description and body), clerk-setup and clerk-cli route migration requests here. clerk-cli adds the migrate consent gates to its agent-mode notes. The README lists the skill. The plugin finds it in skills/ on its own, so no manifest changes.

What the agent must do:

• Run the dry run and relay its checks. Import or undo only after the user says yes to them, in a later turn.
• Never pass --yes, --include-signed-in, --reserve-unverified, --skip-legal-checks or --allow-partial without the user's yes.
• Name the source instance when exporting from Clerk. In agent mode the CLI refuses until it is named, because the linked project is usually the destination.
• Before writing a custom source, ask six questions (email and phone verification, password hasher, file shape, metadata, account state) and check each answer against the file.

Deadline

• Merge after feat(migrate): make clerk migrate generally available cli#531 ships in a clerk release. That PR removes the CLERK_EXPERIMENTAL=migrate gate, and a merge here publishes the skill to the public registry right away. do not merge is applied until then.

Other resources

• CLI stack: feat(migrate): add migration command suite cli#479 (tracking), slices feat(migrate): add the hidden migrate gate and map Clerk and Supabase files cli#517–#520, #515, #524–#531
• Follow-ups outside this repo: add clerk-migrate to DEFAULT_UPSTREAM_SKILLS in clerk/cli so clerk init installs it, and to the skills table in clerk-docs

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