refactor: codebase audit cleanup — dead code, dedup, module splits

Multi-agent audit + aggressive cleanup across server/web/coder/booterm,
delivered behind a DEFER discipline so none of the in-flight files were
touched. Removes dead code/deps/columns, dedups server + coder helpers,
and splits the oversized modules (tools.ts, opencode-server.ts,
sentinel-summaries, turn.ts, TerminalPane.tsx) behind stable contracts.
Adds 78 parity/unit tests (server 587, coder 323); fixes two latent bugs
(ChatPane queue keys, FileViewerOverlay blank-line parity).

Intended tag: v2.7.12-audit-cleanup.

apps/web/src/pages/Session.tsx

@@ -61,7 +61,6 @@ function SessionInner({ sessionId }: { sessionId: string }) {
     initializeFirstChatIfEmpty,
     validatePanes,
   } = panesHook;
-  const [coderConnected, setCoderConnected] = useState<Record<string, boolean>>({});
   const activePane = panes[activePaneIdx];
   const activeIsCoder = activePane?.kind === 'coder';
 
@@ -522,11 +521,6 @@ function SessionInner({ sessionId }: { sessionId: string }) {
           session={session}
           project={project}
           onAddPane={addPaneAndSwitch}
-          onCoderConnectedChange={(paneId, connected) =>
-            setCoderConnected((prev) =>
-              prev[paneId] === connected ? prev : { ...prev, [paneId]: connected },
-            )
-          }
         />
       )}
     </div>