Remove dead code: old context builder, plan_context, journal parsing

Removed from context.rs: ContextPlan, plan_context,
render_journal_text, assemble_context, truncate_at_section,
find_journal_cutoff, parse_msg_timestamp. All replaced by
trim_conversation + journal from memory graph.

Removed from tui.rs: most_recent_file, format_duration
(filesystem scanning leftovers).

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

src/subconscious/hook.rs

@@ -6,7 +6,6 @@
 
 use std::collections::HashSet;
 use std::fs;
-use std::fs::File;
 use std::io::Write;
 use std::path::Path;
 use std::process::Command;