Remove the deprecated ug configure skills command

[xsh310]

What changed and why

ug configure skills is now fully covered by the ug skills commands, so this removes it:

• a bare ug skills registers the schema-less skills MCP connection (what ug configure skills with no --location did);
• ug skills add / ug skills remove (with or without --mcp) manage MCP scopes and downloads (what ug configure skills --location / --skill / --mcp did).

Removed:

• the ug configure skills command in cli.py;
• its now-dead configure_skills_mcp_command helper in mcp.py;
• the tests that covered them (TestConfigureSkillsCommand, TestConfigureSkillsMcpCommand).

Also repointed the ug status note and two comments that named the old command. ug configure itself is unchanged (bare ug configure still runs via its callback); ug configure skills now reports "No such command".

How do you know it works?

• uv run ruff format --check and uv run ruff check are clean, and the unit suite passes (the one failing test is the pre-existing live-gateway e2e case, unrelated to this change).
• ug configure --help renders, ug configure skills returns "No such command 'skills'", and the ug skills surface is unchanged.

Stack

Stacked on #729 (its base is that PR's branch); depends on #729 landing first.

This pull request and its description were written by Isaac.

[sunishsheth2009]

Don't we need to update any ReadMe file?

[xsh310]

Good question. No README change is needed here: the README never documented ug configure skills, so removing it leaves nothing stale (rg -i 'configure skills' README.md returns nothing). The skills workflow the README documents is the ug skills family (bare ug skills, ug skills add, ug skills remove, ug skills list), and that section was already refreshed in #729 (the PR directly below this one in the stack), including the new bare ug skills entry.