delete dead flat-file journal code from thought/context.rs

Journal entries are loaded from the memory graph store, not from the
flat journal file. Remove build_context_window, plan_context,
render_journal_text, assemble_context, truncate_at_section,
find_journal_cutoff, parse_journal*, ContextPlan, and stale TODOs.
Keep JournalEntry, default_journal_path (write path), and the live
context management functions. -363 lines.

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

src/agent/runner.rs

@@ -982,9 +982,6 @@ impl Agent {
             &self.client.model,
             &self.tokenizer,
         );
-        // Don't overwrite journal — it was loaded from the memory graph
-        // in load_startup_journal. The old build_context_window reads
-        // from a stale flat file. TODO: remove build_context_window.
         self.context.entries = messages.into_iter()
             .map(ConversationEntry::Message).collect();
         self.last_prompt_tokens = 0;