mecatui: `/migrate` — render the arrival report in the zero state, and offer to finish the job conversationally

mecatui: /migrate — render the arrival report in the zero state, and offer to finish the job conversationally

Sub-issue of the portability epic (#410). Depends on sub-issue A (the scan/report); this issue is
presentation plus one follow-up-session seam, and should add no discovery logic of its own.

Problem

A developer's first contact with mecatl is mecatui's zero state. If they arrived from Claude
Code, that is precisely the moment their existing setup is invisible to them — and the moment
a CLI subcommand they've never heard of cannot help.

Proposal

• First-run hint. When ~/.claude (or a workspace CLAUDE.md/.claude/) exists and no
  mecatl-native config does, the zero state
  (cmd/mecatui/ui/zerostate*) surfaces a single line offering the report.
  Once dismissed, it stays dismissed — this is a hint, not a wizard.
• /migrate slash command. Registered in the caps-gated palette alongside
  /mcp, /agents, /skills (see docs/tui.md — the built-in command list is locked by a
  test, so that test moves with this change). Renders sub-issue A's three buckets as a
  scrollable overlay, same modal discipline as the existing /agents and /skills views.
• Follow-up session seam. An action on the overlay opens a new session seeded with the
  "not supported / needs a flag" lines as its prompt, so the agent itself walks the user
  through the remainder — Codex's follow-up-thread trick, and the one part of their flow worth
  copying wholesale. The seeded prompt is text the user can read and edit before sending, not
  an auto-run.

Non-goals

• No discovery, resolution or bucket logic — it consumes A's result. If the TUI and the CLI can
  disagree about what was found, that's the defect.
• No writes from the overlay in v1. Enabling a flag or copying skills stays a deliberate CLI
  action; the overlay tells the user the exact command.
• No onboarding wizard, no multi-step flow, no blocking modal on startup.

Acceptance

• With a fixture ~/.claude present, the zero state shows the hint; with none, it does not, and
  the zero state is byte-identical to today's.
• /migrate renders every bucket from A, scrolls, and dismisses like the other overlays; the
  locked built-in-command test is updated to include it.
• The follow-up session is seeded with an editable prompt and is not auto-submitted.
• The overlay writes nothing (asserted by test).
• docs/tui.md updated: the command list, the palette table, and the overlay's key handling.

Start in cmd/mecatui/ui/zerostate* and compare the existing /agents and /skills overlays, then review docs/tui.md and the locked built-in-command test. Use sub-issue A's three-bucket result as the input. Done means the zero-state hint, scrollable /migrate overlay, editable unsent follow-up prompt, updated command and key-handling documentation, and tests proving no writes.