mcp: JSON-RPC client + ffi/curl status_code; PHASE0 §4 amended

First commit of Phase 2 per docs/PHASE2.md §12. Three changes bundled:

mcp.lua (new, 153 lines):
  - M.connect(url, opts) returns a Session.
  - Session:initialize() round-trips initialize + notifications/initialized
    + tools/list. Caches tools for session lifetime (lmcp announces
    capabilities.tools.listChanged = false; no refetch).
  - Session:list_tools() returns the cached tool list.
  - Session:call_tool(name, args) returns (result_table, kind) where
    kind ∈ {"ok", "handler_error", "rpc_error", "transport_error"} per
    the §4 error split. Folded HTTP-level failure into transport_error.
  - Per-server Bearer auth via opts.auth_token or opts.auth_env env-var
    indirection.
  - Captures protocolVersion mismatch as a warning string rather than
    aborting (lmcp doesn't negotiate — N3 in review).

ffi/curl.lua extension:
  - Add curl_easy_getinfo to ffi.cdef.
  - Pre-cast as getinfo_long; helper get_response_code() fetches
    CURLINFO_RESPONSE_CODE (decimal 2097154 = CURLINFOTYPE_LONG | 2).
  - M.post now returns (body, status_code) on transport success;
    (nil, errmsg) on libcurl failure stays unchanged. Phase 1 callers
    reading only the first slot are unaffected.

docs/PHASE0.md §4:
  - Insert `mcp.lua` between broker.lua and router.lua per PHASE2.md §9.
  - Module-stability invariant clarified: rename prohibition is what
    matters; adding new files is additive.

Smoke-test passes for all four kinds against boltzmann lmcp v0.5.4:
  - initialize: ok (7 tools cached)
  - list_dir /tmp: ok (1.2KB content)
  - read_file /nonexistent: ok (boltzmann's baseline §3 quirk —
    isError:false even on failure; content is authoritative)
  - nope_tool: rpc_error (code=-32601)
  - wrong auth: transport_error (HTTP 401)
  - unreachable host: transport_error (DNS failure)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/PHASE0.md

@@ -59,6 +59,7 @@ aish/
 ├── main.lua              # Entry point: arg parsing, config load, REPL start
 ├── repl.lua              # Readline loop, input dispatch, prompt rendering
 ├── broker.lua            # llama.cpp HTTP client; Phase 0: blocking POST
+├── mcp.lua               # MCP JSON-RPC 2.0 client (Phase 2; added 2026-05-12)
 ├── router.lua            # Task classifier: shell / AI / meta
 ├── executor.lua          # Command execution; Phase 0: io.popen
 ├── context.lua           # In-memory conversation history, token budget
@@ -73,7 +74,7 @@ aish/
     └── libc.lua          # Shared: errno, signal, write, read, misc
 ```
 
-All modules are required explicitly from `main.lua`. No module autoloading. File names are stable across phases — later phases fill in bodies, not rename files.
+All modules are required explicitly from `main.lua`. No module autoloading. File names are stable across phases — later phases fill in bodies, not rename files. Adding new files is permitted and additive (e.g. `mcp.lua` was inserted at Phase 2 per docs/PHASE2.md §9); the rename prohibition is what keeps cross-phase wiring stable.
 
 ---