[Feature Request] Add source indicators and grouping for skills in `/` menu

Bug Description

Problem

As the number of skills grows (custom skills, built-in commands, MCP-provided skills), there's no way to distinguish their origin in the / menu. This makes it harder to navigate and find the right skill quickly.

Current state

• All skills appear in a flat list when typing /
• No visual indicator of where a skill comes from (custom vs built-in vs MCP)
• No way to group, tag, or categorize skills
• The only workaround is manual naming conventions (e.g., prefixing with mcp- or oc-)

Proposed solution

1. Source indicator — Show the origin of each skill in the list (e.g., [custom], [mcp:linear], [built-in])
2. Grouping/categories — Allow skills to be grouped by source or custom category in the / menu
3. Optional: custom metadata — Support a category or group field in SKILL.md frontmatter for user-defined organization

Why this matters

Power users who integrate multiple MCP servers and create many custom skills end up with 15+ skills in the menu. Without categorization, finding the right one requires memorizing names. A simple source label would go a long way.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.50
• Feedback ID: 29f5af75-a073-48b7-8b33-a91d52513dca

Errors

[{"error":"Error: NON-FATAL: Lock acquisition failed for /Users/kotaro/.local/share/claude/versions/2.1.50 (expected in multi-process scenarios)\n    at OzR (/$bunfs/root/claude:2600:2098)\n    at SbA (/$bunfs/root/claude:2600:1209)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-23T09:17:06.246Z"},{"error":"Error: LSP server plugin:rust-analyzer-lsp:rust-analyzer crashed with exit code 1\n    at <anonymous> (/$bunfs/root/claude:2551:21471)\n    at emit (node:events:98:22)\n    at #handleOnExit (node:child_process:520:14)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-23T09:17:07.779Z"},{"error":"Error: Request was aborted.\n    at makeRequest (/$bunfs/root/claude:301:3940)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-23T10:05:30.559Z"}]