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/SessionLandingPage.tsx

@@ -12,6 +12,7 @@ import {
 } from '@/components/ui/dialog';
 import { api } from '@/api/client';
 import type { Chat } from '@/api/types';
+import { formatRelative } from '@/lib/format';
 
 interface Props {
   projectId: string;
@@ -34,19 +35,6 @@ interface Props {
   onDeleteChat: (chatId: string) => Promise<void>;
 }
 
-function formatRelative(iso: string): string {
-  const then = new Date(iso).getTime();
-  if (Number.isNaN(then)) return '';
-  const s = Math.max(0, Math.round((Date.now() - then) / 1000));
-  if (s < 60) return 'just now';
-  const m = Math.round(s / 60);
-  if (m < 60) return `${m}m ago`;
-  const h = Math.round(m / 60);
-  if (h < 24) return `${h}h ago`;
-  const d = Math.round(h / 24);
-  if (d < 7) return `${d}d ago`;
-  return new Date(iso).toLocaleDateString();
-}
 
 function byRecent(a: Chat, b: Chat): number {
   return (b.updated_at ?? '').localeCompare(a.updated_at ?? '');