tools: delete ToolDef and FunctionDef

ToolDef and FunctionDef are gone. Tool definitions are static strings
on the Tool struct. The API layer builds JSON from Tool::to_json().

- ChatRequest.tools is now Option<serde_json::Value>
- start_stream takes &[Tool] instead of Option<&[ToolDef]>
- openai::stream_events takes &serde_json::Value for tools
- memory_and_journal_tools() returns Vec<Tool> for subconscious agents
- Subconscious agents filter by t.name instead of t.function.name

No more runtime JSON construction for tool definitions.
No more ToolDef::new(). No more FunctionDef.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/mod.rs

@@ -306,10 +306,9 @@ impl Agent {
                     top_p: me.top_p,
                     top_k: me.top_k,
                 };
-                let tool_defs: Vec<_> = me.tools.iter().map(|t| t.to_tool_def()).collect();
                 me.client.start_stream(
                     &api_messages,
-                    Some(&tool_defs),
+                    &me.tools,
                     ui_tx,
                     &me.reasoning_effort,
                     sampling,