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/AgentPicker.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useState } from 'react';
+import { formatAgo } from '@/lib/format';
 import { Check, ChevronDown } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
@@ -191,12 +192,3 @@ function formatK(n: number): string {
   return `${Math.round(n / 1000)}k`;
 }
 
-function formatAgo(iso: string): string {
-  const then = new Date(iso).getTime();
-  if (Number.isNaN(then)) return '—';
-  const diff = Date.now() - then;
-  if (diff < 60_000) return 'just now';
-  if (diff < 3_600_000) return `${Math.round(diff / 60_000)}m ago`;
-  if (diff < 86_400_000) return `${Math.round(diff / 3_600_000)}h ago`;
-  return `${Math.round(diff / 86_400_000)}d ago`;
-}