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.

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "mantine": {
+      "command": "npx",
+      "args": ["-y", "@mantine/mcp-server"]
+    }
+  }
+}