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-daemon/src/idle.rs

@@ -276,7 +276,7 @@ impl State {
     /// Called when a notification arrives via module channel.
     /// Only injects into tmux when idle — if there's an active session
     /// (recent user or response), the hook delivers via additionalContext.
-    pub fn maybe_prompt_notification(&mut self, ntype: &str, urgency: u8, message: &str) {
+    pub fn maybe_prompt_notification(&mut self, ntype: &str, urgency: u8, _message: &str) {
         if self.kent_present() {
             return; // hook will deliver it on next prompt
         }