feat(soul): MCP tool-bridge — suspend agentic loop for client-executed tools

When handle_chat_agentic hits a tool the soul cannot run in-process (an MCP
connector/plugin surfaced by the Kotlin desktop app), instead of returning
"unknown tool" it now suspends the agentic loop and returns a tool_pending
envelope so the CLIENT executes the tool and posts the result back. Built-in
tools (read_file/write_file/web_get/search_memory/run_command) and Anthropic's
native web_search are unchanged.

Client contract:
- Soul returns (HTTP 200) on an unknown tool:
    { "tool_pending": true, "session_id": "br-...", "call_id": "<tool_use_id>",
      "tool_name": "...", "tool_input": { ... }, "model": "...",
      "agentic": true, "tools_used": [...] }
- Client runs the MCP tool, then POSTs to
    /api/sessions/{session_id}/tool_result
  with body:
    { "call_id": "<the call_id from the envelope>",
      "content": "<MCP tool output as a string>" }
- Soul resumes the loop and returns the same envelope shape: either a final
    { "reply": ..., "tools_used": [...] }
  or another tool_pending if the continuation needs a further MCP tool
  (fully chainable). Saved continuation is one-shot (cleared on resume).

elc-verified (--target=c, exit 0, no stderr) on chat.el, routes.el, and the
full soul.el import graph. Needs Will's build to ship.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

routes.el

@@ -305,6 +305,16 @@ fn handle_request(method: String, path: String, body: String) -> String {
     }
 
     if str_eq(method, "POST") {
+        // MCP tool-bridge resume: POST /api/sessions/{id}/tool_result
+        // The client executed a tool the soul could not run in-process (an MCP
+        // connector/plugin) and posts the result back here so the agentic loop
+        // continues. {id} is the session_id from the prior tool_pending envelope.
+        if str_starts_with(clean, "/api/sessions/") && str_ends_with(clean, "/tool_result") {
+            let after: String = str_slice(clean, 14, str_len(clean))
+            let slash: Int = str_index_of(after, "/")
+            let session_id: String = if slash < 0 { after } else { str_slice(after, 0, slash) }
+            return handle_tool_result(session_id, body)
+        }
         if str_eq(clean, "/imprint/contextual") {
             return route_imprint_contextual(body)
         }