v0.5.0: route the neovim MCP through MetaMCP

mcp-neovim-server no longer runs as a per-session npx stdio child.
It is now a MetaMCP namespace upstream on emmett (one persistent
process), reached over Streamable-HTTP via the mcp-session-pool:

  Claude Code -> pool.localhost:12010/p/neovim/mcp
    -> MetaMCP neovim namespace -> mcp-neovim-server -> nvim socket

- .mcp.json: stdio npx server -> streamable-http pool endpoint
- agents/companion.md + skills: tool names are now 3-segment
  (mcp__neovim__neovim__vim_*) — the MetaMCP aggregation shape; the
  surface description and unavailability runbook updated for the
  three-link path (nvim socket / MetaMCP / session)
- README: connection-path diagram

Host side (servers/emmett, deployed separately): neovim added to
services.mcp-session-pool.upstreams; metamcp.nix seeds the neovim
mcp_servers row + mapping and adds nodejs to the unit PATH.

.mcp.json

@@ -1,11 +1,8 @@
 {
 	"mcpServers": {
 		"neovim": {
-			"command": "npx",
-			"args": ["-y", "mcp-neovim-server"],
-			"env": {
-				"NVIM_SOCKET_PATH": "/run/user/1000/nvim.sock"
-			}
+			"url": "http://pool.localhost:12010/p/neovim/mcp",
+			"type": "streamable-http"
 		}
 	}
 }