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-memory/src/store/persist.rs

@@ -16,7 +16,7 @@ use capnp::serialize;
 
 use std::collections::HashMap;
 use std::fs;
-use std::io::{BufReader, BufWriter, Seek};
+use std::io::{BufReader, Seek};
 use std::path::Path;
 
 impl Store {
@@ -841,7 +841,7 @@ pub fn strip_md_keys() -> Result<(), String> {
 // back up first, preserve history, and never write from a potentially
 // incomplete in-memory snapshot.
 #[allow(dead_code)]
-fn _rewrite_store_DISABLED(_store: &Store) -> Result<(), String> {
+fn _rewrite_store_disabled(_store: &Store) -> Result<(), String> {
     panic!("rewrite_store is disabled — see comment above");
 }