refactor(mcp): split claude launch-config layer into mcp_config.rs

docs/conventions.md

@@ -360,7 +360,8 @@ the var is absent or empty.
 **Updating the surface** — when a new `#[tool]` fn is added to `HiveServer`
 in `hive-ag3nt/src/mcp.rs`, add its name to the matching `ToolGroup::tools()`
 slice in `hive-sh4re/src/lib.rs`. That's the single source of truth;
-`allowed_mcp_tools` reads it at session start.
+`mcp_config::allowed_mcp_tools` (in `hive-ag3nt/src/mcp_config.rs`) reads it at
+session start.
 
 ## Capabilities
 

docs/turn-loop.md

@@ -743,7 +743,7 @@ than deriving from SSE events.
 runs the body, logs the result. Pre-/post-log only — the inbox
 status hint moved to the wake prompt + UI header.
 
-### Tool whitelist (`mcp::ALLOWED_BUILTIN_TOOLS`)
+### Tool whitelist (`mcp_config::ALLOWED_BUILTIN_TOOLS`)
 
 - Allowed built-ins: `Edit`, `Glob`, `Grep`, `Read`, `Write`.
 - Tool-group-gated built-ins: `WebFetch`, `WebSearch` (added when the