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/agent.rs

@@ -1066,15 +1066,15 @@ struct ContextPlan {
     /// Index into recent conversation: skip messages before this
     conv_trim: usize,
     /// Total recent conversation messages
-    conv_count: usize,
+    _conv_count: usize,
     /// Tokens used by full journal entries
-    full_tokens: usize,
+    _full_tokens: usize,
     /// Tokens used by header-only journal entries
-    header_tokens: usize,
+    _header_tokens: usize,
     /// Tokens used by conversation (after trimming)
-    conv_tokens: usize,
+    _conv_tokens: usize,
     /// Total budget available (after identity, memory, reserve)
-    available: usize,
+    _available: usize,
 }
 
 /// Build a context window from conversation messages + journal entries.
@@ -1233,11 +1233,11 @@ fn plan_context(
         full_start,
         entry_count: entries.len(),
         conv_trim,
-        conv_count: recent.len(),
-        full_tokens: full_used,
-        header_tokens: header_used,
-        conv_tokens: trimmed_conv,
-        available,
+        _conv_count: recent.len(),
+        _full_tokens: full_used,
+        _header_tokens: header_used,
+        _conv_tokens: trimmed_conv,
+        _available: available,
     }
 }