ARC skill: mcp

MCP is a transport into the same AFS world, not a second ontology.

Capability negotiation (local first)

Decide which MCP path is available before assuming tools exist:

Host situation	What to do
`arc` CLI is on `PATH`	Use local stdio: plugin `.mcp.json` server `arc-local` → `arc mcp` (bridge to the daemon; starts it if needed).
`arc` CLI is not on `PATH`	Plugin skills still load. Do not treat this as a silent total failure. Install the CLI (`curl -fsSL https://arc.afsd.io/install.sh \| bash` — see `docs/guides/arc-cli-distribution.md`). Plugin bootstrap that auto-installs CLI is S5 (not shipped yet).
Want remote / no-shell host	Not ready in v1. `arc-remote` is intentionally absent from `.mcp.json` so "not configured" is not the same colour as "endpoint down". Tracked in #6432 (hosted endpoint + OAuth grant).

Say the remote path is unready out loud. An agent that reads "remote unavailable" behaves differently from one that never sees remote mentioned.

Distinguish failure reasons when local MCP will not start:

arc missing — command not found / spawn ENOENT for arc. Fix: install CLI (above). Message shape: needs arc on PATH.
arc present but daemon unreachable — CLI runs; bridge cannot attach / daemon will not stay up. Fix: arc service start (or check instance). Message shape: daemon unavailable — not "arc missing".