Web UI: add Gemini CLI as a configure host

[hyoshi]

Summary

mureo setup gemini already exists at the CLI level (mureo/cli/setup_gemini.py) and registers mureo for Gemini CLI. The configure Web UI does not support Gemini — SUPPORTED_HOSTS = ("claude-code", "claude-desktop") only, and the wizard host step offers just Claude Code / Claude Desktop. This issue tracks extending the Web UI to Gemini.

Current state

CLI (setup_gemini.py) registers mureo as a Gemini CLI extension:

• ~/.gemini/extensions/mureo/gemini-extension.json — JSON with mcpServers.mureo + contextFileName: CONTEXT.md. Merges only mureo's keys; other extensions under ~/.gemini/extensions/ are untouched.
• No PreToolUse-style guard surface in Gemini CLI → no credential-guard step (unlike Claude/Codex).
• Custom slash commands are .toml (different from mureo's bundled .md) → currently out of scope in the CLI helper too.

The Web UI knows none of this.

Work required

Gemini needs its own registration/detection driver in the generalized host layer:

• host_paths.py: add a gemini host pointing at ~/.gemini/extensions/mureo/gemini-extension.json (the MCP registry) + skills/credentials paths. It IS JSON (unlike Codex TOML) but the structure differs from Claude's root mcpServers — detection in status_collector.py must read the extension file, not ~/.claude.json.
• SUPPORTED_HOSTS + wizard host step (buildStepBody_host, i18n) + wizard.js host options.
• setup_actions.py: host branch calling the Gemini extension install/remove primitives instead of claude mcp. The Gemini branch must skip the credential-guard step (no hook surface) — mirror the existing Claude-Desktop "unsupported_on_desktop" no-op pattern.
• status_collector.py: detect installed mureo / official providers from the Gemini extension JSON.

Decisions required

• Official providers (google-ads-official etc.) on Gemini: in scope for the Web UI, or mureo-MCP-only? (Ties into Phase 2: Credential supply for official MCP providers (google-ads-official, ga4-official) #102.)
• Gemini extension model is one extension dir per tool — confirm whether official providers become separate extensions or entries under mureo's.
• Skills/commands .md → Gemini .toml conversion: in scope here or a separate follow-up (already noted as deferred in setup_gemini.py).

Acceptance criteria

• Wizard offers Gemini as a host; selecting it writes ~/.gemini/extensions/mureo/gemini-extension.json correctly and idempotently, preserving other extensions.
• The credential-guard step is a clean no-op for Gemini (documented, not an error).
• Dashboard status reflects what is actually registered in the Gemini extension file.
• Bulk-clear / remove unregister from the Gemini extension dir only (no cross-talk with ~/.claude.json / ~/.codex).
• Parity with the CLI mureo setup gemini; validated against the real extension file, not only unit tests.

References

• mureo/cli/setup_gemini.py (existing CLI primitive to reuse).
• Web UI host layer: mureo/web/host_paths.py, status_collector.py, setup_actions.py, mureo/_data/web/wizard.js, i18n.json.
• Related: feat: Browser-based config UI for non-engineer onboarding #88 (config UI), Phase 2: Credential supply for official MCP providers (google-ads-official, ga4-official) #102 (Phase 2 official-provider credentials). Sibling issue: Codex CLI Web UI host.