Add ug skill status for downloaded and MCP skill state

[xsh310]

What did you change, and why?

Change: Add a new ug skill status command that reports both sources of configured skills in one place: downloaded skills (from the ~/.ucode/skills.json manifest) and the skills MCP scope (from state.json). It offers a human report and a --json mode, plus a --path filter for the downloaded listing. ug status now reuses the same collector for a two line skills summary that points at the new command.

Why: ug skill remove accepts --skills/--location/--agents for non interactive use, but an agent had no reliable way to discover what is currently configured to fill those flags. ug status showed only the MCP scope, as prose, with no machine readable output, and downloaded skills were absent entirely. ug skill status --json closes that gap with a stable schema whose fields map directly onto the removal flags (fqn to --skills, schema to --location, project base to --path, by_agent keys to --agents).

Design doc: ai-gateway/scripts/ug_skill_status_design.md (in the universe repo).

Notable behavior:

• Grouping and order: downloaded skills are grouped by base and sorted by FQN, so every skill from one <catalog>.<schema> sits together.
• Manifest reconciliation: collecting prunes any download record whose on disk skill directory was deleted out of band (via skills_state.forget), so the listing only shows skills that are actually installed. Pruning touches only the manifest, never on disk files or state.json. The dirs exist check moved from skills_download to skills_state so the removal picker and status share one definition.

How do you know it works?

Testing:

• New tests/test_skills_status.py drives collect/to_json/render off a fabricated manifest and MCP state (no network), covering: FQN sort and schema grouping; pruning a record with any missing directory while keeping present ones and leaving on disk files alone; --path filtering the downloaded side while still reconciling globally; shared MCP scope collapsing in the human view but staying per agent in JSON; divergent per agent scopes; the JSON field to flag mapping; surfacing an origin workspace that differs from the configured one; and empty/unconfigured states.
• Existing ug status skills tests still pass unchanged (the MCP scope labels are preserved).
• Full suite: uv run pytest green except two failures that are pre existing on the base branch and unrelated to this change (test_claude_smart_routing_v2 PTY test and test_e2e_user_agent). uv run ruff check and uv run ruff format --check are clean.
• Manual e2e of ug skill status, ug skill status --json, and the ug status summary against an isolated home directory.

This pull request and its description were written by Isaac.