cleanup: fix all build warnings, delete dead DMN context code

- Delete poc-daemon/src/context.rs dead code (git_context, work_state,
  irc_digest, recent_commits, uncommitted_files) — replaced by
  where-am-i.md and memory graph
- Remove unused imports (BufWriter, Context, similarity)
- Prefix unused variables (_store, _avg_cc, _episodic_ratio, _message)
- #[allow(dead_code)] on public API surface that's not yet wired
  (Message::assistant, ConversationLog::message_count/read_all,
  Config::context_message, ContextInfo fields)
- Fix to_capnp macro dead_code warning
- Rename _rewrite_store_DISABLED to snake_case

Only remaining warnings are in generated capnp code (can't fix).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-agent/src/ui_channel.rs

@@ -63,7 +63,9 @@ pub struct ContextInfo {
     pub available_models: Vec<String>,
     pub prompt_file: String,
     pub backend: String,
+    #[allow(dead_code)]
     pub instruction_files: Vec<(String, usize)>,
+    #[allow(dead_code)]
     pub memory_files: Vec<(String, usize)>,
     pub system_prompt_chars: usize,
     pub context_message_chars: usize,