chore: migrate cursor rules to portable agent skills + mantine MCP guidance

* chore: migrate cursor rules to portable agent skills and add mantine MCP guidance

Replace Cursor-only .cursor/rules/*.mdc with Agent Skills open-standard
SKILL.md files in .agents/skills/ (canonical), mirrored to .cursor/skills/
and .claude/skills/ so they work across Claude Code, Cursor, Codex, and
OpenCode. Add mantine MCP server guidance to AGENTS.md and un-ignore
.agents/skills/ in .gitignore.

* refactor: single-source agent skills at .agents/skills

Cursor, Codex, and OpenCode all read .agents/skills/ natively, so the
.cursor/skills/ mirror was redundant. Claude Code reads .claude/skills/,
which is now a symlink back to .agents/skills/ so the content lives once.
Deletes .cursor/skills/ and converts .claude/skills/ to a symlink.

* chore: ship mantine MCP server via project-scoped .mcp.json

Declares the keyless @mantine/mcp-server in .mcp.json (Claude Code
project-scope standard, committable) and documents MCP server setup and
the mantine entry in AGENTS.md.

* chore: address coderabbit security-review and markdown lint findings

Add explicit security-review guidance before exposing MCP tools and label
plain-text code fences to satisfy markdownlint MD040.

AGENTS.md

@@ -84,6 +84,16 @@ When modifying user-facing code, you MUST also update the corresponding document
 - Lint: oxlint (not ESLint)
 - Format: oxfmt (not Prettier)
 - UI: Mantine (not Tailwind) — Tailwind is only used in docs app
+- Mantine: use the `mantine` MCP server (configured in `.mcp.json`; tools `get_item_doc` / `get_item_props` / `search_docs`) for current v9 APIs before writing component code. Prefer built-in primitives (`Combobox`/`useCombobox` for selects, the polymorphic `component` prop, `@mantine/hooks`) over hand-rolled equivalents. Check `packages/ui/` for existing conventions first.
+
+## MCP servers
+
+`.mcp.json` at the repo root declares project-scoped MCP servers (the standard Claude Code layout; other tools can import the same `mcpServers` block). See the `mantine` entry there for the Mantine v9 server, and the upstream docs at https://mantine.dev/guides/llms/ for per-tool setup (OpenCode: `.opencode/opencode.json`, Codex: `.codex/config.toml`). Only keyless server definitions belong in `.mcp.json`; anything needing secrets stays in local dotfiles.
+
+## Agent Skills
+
+Portable agent skills live in `.agents/skills/` (the Agent Skills open standard). Cursor, Codex, and OpenCode read `.agents/skills/` natively. Claude Code does not scan it, so `.claude/skills/` is a symlink back to `.agents/skills/`. Install skills at `.agents/skills/<name>/SKILL.md` — that single copy is used everywhere. See `.agents/skills/` for available skills (documentation-sync, mcp-integration, codebase-context, datatable-migration).
+
 - Icons: @tabler/icons-react
 - Docs app can import from `@homarr/definitions` for shared types
 - Run `pnpm dev:cli -- dev` to browse local `homarr:*` images and remote PR images.