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/api/mod.rs

@@ -14,7 +14,7 @@
 mod anthropic;
 mod openai;
 
-use anyhow::{Context, Result};
+use anyhow::Result;
 use reqwest::Client;
 use std::time::{Duration, Instant};
 

poc-agent/src/api/openai.rs

@@ -62,7 +62,7 @@ pub async fn stream(
     let mut empty_deltas: u64 = 0;
     let mut first_content_at: Option<Duration> = None;
 
-    let reasoning_enabled = reasoning_effort != "none";
+    let _reasoning_enabled = reasoning_effort != "none";
 
     while let Some(event) = reader.next_event(&mut response).await? {
         // OpenRouter sometimes embeds error objects in the stream