IT BUILDS: Full AST migration compiles — zero errors

All callers migrated from old context types to AstNode/ContextState.
Killed: Message, Role (api), ConversationEntry, ContextEntry,
ContextSection, working_stack, api/parsing.rs, api/types.rs,
api/openai.rs, context_old.rs.

Oneshot standalone path stubbed (needs completions API rewrite).
12 warnings remaining (dead code cleanup).

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

src/agent/mod.rs

@@ -577,7 +577,6 @@ impl Agent {
         self.push_node(AstNode::tool_result(&output));
     }
 
-
     pub fn conversation_from(&self, from: usize) -> &[AstNode] {
         let conv = self.context.conversation();
         if from < conv.len() { &conv[from..] } else { &[] }
@@ -698,7 +697,4 @@ impl Agent {
     pub fn client_clone(&self) -> ApiClient {
         self.client.clone()
     }
-
-
-
 }