feat(settings): Customized hub for MCP servers & skills

[Legacynnn]

Summary

A new Settings → Customized panel: a hub to browse and one-click install curated MCP servers and skills, globally, for Claude and Codex.

• Browse tab — searchable curated catalog (GitHub, Filesystem, Fetch, Context7, Playwright MCP servers; thermo-nuclear code review, git-commit-writer, pr-description-writer, deep-research skills). Install per provider from a single toggle dialog; takes effect next session.
• Installed tab — collapsible per-provider inventory cards (Claude, Codex, Cursor, GitHub CLI) showing each provider's loaded skills/MCP as a proper grouped list, plus a consolidated Installed overall list deduped across providers.

Backend (write path)

• ~/.claude.json writer — top-level mcpServers (user-scope; the Agent SDK auto-loads it, so global installs work with no sidecar change). Round-trips through serde_json::Value, atomic write.
• ~/.codex/config.toml writer — [mcp_servers] via toml_edit, preserving comments and unknown keys. Atomic write.
• Generalized the npx skills add machinery (arbitrary source + agents, always -g); thermo-nuclear skill ships as Helmor-vendored, provider-adapted copies embedded via include_str!.
• Claude config inspection now also scans ~/.claude.json so global installs surface in the inventory.

Test plan

• Rust: cargo test --tests — 1702+ pass incl. 9 new config-writer tests (insert / idempotent / remove / preserve unknown keys / preserve TOML comments).
• Sidecar: bun test — 449 pass; new cases assert top-level (global) mcpServers are not pulled into the project-injection path.
• Frontend: new use-customized-data tests (join + installed-overall derivation) pass; typecheck + biome clean.
• Manual: open Settings → Customized in a debug build, install a curated MCP, confirm ~/.claude.json / ~/.codex/config.toml update and tools appear in a new session.

🤖 Generated with Claude Code