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/log.rs

@@ -80,6 +80,7 @@ impl ConversationLog {
     }
 
     /// Count messages in the log without loading content.
+    #[allow(dead_code)]
     pub fn message_count(&self) -> Result<usize> {
         if !self.path.exists() {
             return Ok(0);
@@ -94,6 +95,7 @@ impl ConversationLog {
 
     /// Read all messages from the log. Returns empty vec if log doesn't exist.
     /// NOTE: Don't use this in hot paths — use read_tail() instead.
+    #[allow(dead_code)]
     pub fn read_all(&self) -> Result<Vec<Message>> {
         if !self.path.exists() {
             return Ok(Vec::new());