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.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/components/panes/SettingsPane.tsx

@@ -266,9 +266,6 @@ function SessionSection({ session, project }: { session: Session; project: Proje
             Inherit project default ({project.default_web_search_enabled ? 'on' : 'off'})
           </label>
         </div>
-        <p className="text-xs text-muted-foreground italic">
-          Plumbed for Batch 8 (web_search tool). No effect yet.
-        </p>
       </div>
 
       <AllowedReadPathsSection session={session} />
@@ -532,7 +529,7 @@ function ProjectSection({ project }: { project: Project }) {
           />
         </div>
         <p className="text-xs text-muted-foreground italic">
-          Applies to new sessions only. Plumbed for Batch 8.
+          Applies to new sessions only.
         </p>
       </div>