feat(install): recommend cli surface + update --surface mcp↔cli migration

[HumanBean17]

Two setup-wizard fixes building on the recently-shipped jrag CLI.

1. update can now switch surfaces (mcp ↔ cli)

• New update --surface {mcp,cli} flag. On a TTY with no flag it prompts (cursor on the current surface, so Enter keeps it); non-interactive update without --surface refreshes each host on its own recorded surface and migrates nothing.
• A switch migrates each affected host: tears down the old surface's artifacts (_remove_mcp_entry pops only the java-codebase-rag server, preserving siblings; old skill/agent files removed), deploys the new surface's, and rewrites the .java-codebase-rag.hosts marker so it persists.
• Fails fast (exit 1) if the target binary is missing; dry-run safe.

2. Install recommends the cli surface

• select_surface lists cli (Recommended) first and defaults to cli everywhere (interactive cursor + non-interactive default).
• Fixed prompt() so questionary.select(default=...) actually positions the cursor: questionary validates default only against Choice.value, so dict choices are now normalized to Choice before the call.

⚠️ Breaking change

Non-interactive install/update without --surface now yields cli (previously mcp). CI that relied on the implicit MCP default must add --surface mcp.

Review-driven correction

Code review caught that the non-TTY no-flag update path returned the first host's surface as the global target, so a mixed-surface marker would get normalized. Fixed: that path now refreshes each host on its own surface and migrates nothing. Covered by test_update_no_flag_non_tty_mixed_marker_does_not_migrate and test_update_surface_normalizes_mixed_marker.

Verification

• tests/test_installer_surface.py — 31 passed (incl. new migration/mixed-surface/user-scope/prompt-spy cases)
• tests/test_installer.py — 85 passed; tests/test_java_codebase_rag_cli.py update-flag tests pass
• Real pty smoke tests: fresh install renders » cli (Recommended) first → Enter picks cli; update renders cli (Recommended) first with cursor on the current surface → Enter keeps it
• Full suite: 1092 passed / 15 skipped / 1 environmental failure (test_cli_lifecycle_stdout_invariant_erase_quiet — .venv/bin not on PATH in the sandbox subprocess; passes with the venv on PATH, unrelated to this change)

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