Add skill remove --mcp for global scope removal

[xsh310]

What

Adds ucode skill remove --mcp, the inverse of ucode skill add --mcp. It interactively removes skill schemas from the skills MCP connection across every configured agent. Removing downloaded skills is not supported yet, so the command requires --mcp.

How

• remove_skills_command() reads each configured client's skills scope, offers the union of schemas via a picker (each annotated with the clients that carry it), and removes the selected schemas from every client that has them.
• It reuses the per-client update path, so dropping the last schema leaves the schema-less connection registered rather than deleting it. It never adds or reconfigures anything and needs no Databricks auth.
• cli.py gains the skill remove command and a status hint pointing at ug skill add / ug skill remove --mcp.

Tests

• test_mcp.py: a selected schema is removed from every client and each is re-registered; removing all schemas keeps the schema-less connection; an empty scope is a no-op that never opens the picker; the picker is offered each client's real scope.
• test_cli.py: skill remove without --mcp errors; skill remove --mcp dispatches the global removal.

uv run pytest tests/test_mcp.py tests/test_cli.py tests/test_lint.py is green.

Manual verification (installed build)

Ran against the installed build (0.1.0+91.g5c0dc1e) in the isolated sandbox (temp HOME, stub claude/codex binaries recording registrations, DATABRICKS_BEARER for offline auth), starting from state claude=[claude.only], codex=[shared.skills]. Invoked remove_skills_command() with the interactive picker stubbed to select shared.skills (the picker needs a TTY); all state I/O and per-client registration ran for real.

Check	Result
Picker input	Offered both clients' scopes: {claude:[claude.only], codex:[shared.skills]}
State after	shared.skills dropped from codex; result {claude:[claude.only]} with codex's now-empty entry removed
Registrations	Only codex re-registered, with the schema-less URL; claude untouched
Guard	ucode skill remove (no --mcp) exits with ERROR Removing downloaded skills is not supported yet ...

Stacking

Third in the stacked per-agent skills series, based on xshen/skill-per-agent-status (#523). Per-agent remove builds on this next. Reviewing the diff against that base shows just this change. It rebuilds behavior originally designed by Arthur Jenoudet on the current per-client-map state model.

This pull request and its description were written by Isaac.