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/RightRail.tsx

@@ -274,9 +274,7 @@ export function RightRail({ projectId, sessionId }: Props) {
           path={viewerFile.path}
           content={viewerFile.content}
           lang={inferLanguage(viewerFile.path)}
-          projectId={projectId}
           onClose={() => setViewerFile(null)}
-          onNavigate={(path) => void openFile(path)}
         />
       )}