feat(mcp): serve tools from the Ix CLI instead of shipping a server

[KageBinary]

Replaces this plugin's own MCP server with the one the Ix CLI now ships.

What changed

mcp/server.py was a 468-line FastMCP server whose 23 tools each shelled out to the ix CLI. The CLI serves the same 23 tools itself as ix mcp, so this was a second implementation of one surface — maintained here, and drifting independently of the other five plugins' servers.

--mcp now registers the CLI's server instead of copying one, delegating to ix mcp install --host codex.

Tool parity is exact. ix mcp serves the same 23 names this server did, verified by diffing the two surfaces. Nothing an agent could call before is gone.

Windows

Registration goes through ix mcp install rather than a hand-built codex mcp add, so the launcher is resolved once in the CLI. npm ships no ix.exe — only ix.CMD — and a host spawning the bare name through CreateProcess never consults PATHEXT. That is the failure #13 fixed here; the CLI now makes the same fix once for all seven hosts it knows about.

Version floor

Gated on ix >= 0.9.3, the first release carrying ix mcp. Below that the installer says so and does nothing, rather than writing a registration that cannot start.

This PR cannot land before that release exists. It is opened now so the migration is reviewable, not because it is ready to merge.

Tests

Removed with the server:

• tests/test_mcp_cli_invocation.py (431 lines, drove all 23 tools against a stub ix)
• tests/test_llm_fastpath.py and mcp/ix_llm.py — added by feat(mcp): use --format llm where the installed CLI supports it #23 after this branch was cut. ix_llm.py was imported only by server.py; the CLI does its own --format llm selection internally, so the fast-path is not lost, just no longer this repo's to gate.
• the test-local.sh checks that imported the module to count its tools

Replaced by checks that the installer copies nothing and that the version floor is not below the subcommand. bash test-local.sh passes, and CI is green on ubuntu and windows.

Not in scope

Agents, hooks and skills stay exactly as they are. MCP standardizes tools, not hooks or skills, so they cannot move — see docs/mcp-plugin-consolidation.md in the Ix repo for the full matrix.

[KageBinary]

The version gate is satisfied — ix v0.9.3 is released.

ix mcp is in the published artifact; verified by extracting ix-0.9.3-linux-amd64.tar.gz and running ix mcp --help against it. The Homebrew formula is on v0.9.3 too (it had been stuck on v0.9.1), so brew install ix gets a CLI with the command as well.

Nothing further is needed here — the floor this PR gates on now exists. Ready to merge; the repo ruleset means it needs someone with bypass rights.