skill/mcp remove: normalize --agents aliases like the add paths

[xsh310]

What

skill remove --agents and mcp remove --agents only lowercased the agent names before setup_mcp_clients' configured-client membership check. An alias like claude-code therefore failed with "not configured", even though skill add and mcp add accept it (they resolve aliases through normalize_tool). A user who added a scope with an alias could not remove it with the same name. mcp remove had the identical pre-existing gap.

How

• Add normalize_agent / normalize_agents helpers in agents/__init__.py. normalize_agent is normalize_tool plus the MCP-only cursor agent (which has no model routing, so it is not a normalize_tool value); normalize_agents parses a comma-separated --agents value into canonical names.
• Route all four skill/mcp --agents boundaries (add and remove) through normalize_agents, so add and remove resolve names identically. The add-path helper _configure_agents_for_mcp now uses normalize_agent in place of its inline a if a == "cursor" else normalize_tool(a).
• Leave normalize_tool unchanged, so the launch/configure paths that must reject cursor as a model agent keep that behavior.
• Behavior is identical for valid input; an invalid name now raises the clearer Unsupported tool '...' at parse time instead of a later "not configured".

Tests

• test_agents_init.py: unit tests for normalize_agent (aliases, cursor, invalid raise) and normalize_agents (None, empty, dedupe, normalize).
• test_cli.py: skill remove and mcp remove resolve claude-code / gemini-cli to {claude, gemini}; adds a first forwarding test for mcp remove --agents.
• uv run pytest tests/test_agents_init.py tests/test_cli.py tests/test_mcp.py tests/test_lint.py passes (562); ruff check clean.

Stacking

Follow-up on top of #525 (xshen/skill-per-agent-remove), addressing Sunish's P2 review comment there about normalizing --agents aliases on the remove paths. Also fixes the identical pre-existing gap in mcp remove.

This pull request and its description were written by Isaac.

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