delete 20 dead public functions across 12 files

Removed functions with zero callers: parse_timestamp_to_epoch,
hash_key, search_weighted_debug, extract_query_terms, format_results,
move_to_neighbor, adjust_edge_strength, update_graph_metrics,
nearest_to_seeds, nystrom_project, chat_completion_stream, cmd_read,
context_message, split_candidates, split_plan_prompt,
split_extract_prompt, log_event_pub, log_verbose, rpc_record_hits,
memory_definitions. -245 lines.

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

src/config.rs

@@ -443,17 +443,6 @@ pub struct SessionConfig {
     pub app: AppConfig,
 }
 
-impl SessionConfig {
-    /// Join context parts into a single string for legacy interfaces.
-    #[allow(dead_code)]
-    pub fn context_message(&self) -> String {
-        self.context_parts.iter()
-            .map(|(name, content)| format!("## {}\n\n{}", name, content))
-            .collect::<Vec<_>>()
-            .join("\n\n---\n\n")
-    }
-}
-
 /// A fully resolved model ready to construct an ApiClient.
 #[allow(dead_code)]
 pub struct ResolvedModel {