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.

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
 	"name": "nvim-agentic-companion",
-	"version": "0.4.0",
+	"version": "0.5.0",
 	"description": "Neovim companion that pairs the running editor (via mcp-neovim-server) with the official Claude Code IDE plugin (coder/claudecode.nvim). Provides an opinionated agent and skills for introspecting your live nvim state (buffers, keymaps, diagnostics, plugins), driving it on your behalf (open files, run keymaps, execute lua), and handing work cleanly to the in-editor Claude Code session for buffer edits.",
 	"author": {
 		"name": "oleks",