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/package.json

@@ -15,19 +15,16 @@
     "@xterm/addon-fit": "0.10.0",
     "@xterm/addon-search": "^0.15.0",
     "@xterm/addon-web-links": "0.11.0",
-    "@xterm/addon-webgl": "^0.19.0",
     "@xterm/xterm": "5.5.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^1.16.0",
-    "next-themes": "^0.4.6",
     "radix-ui": "^1.4.3",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^10.1.0",
     "react-router-dom": "^6.26.0",
     "remark-gfm": "^4.0.1",
-    "shadcn": "^4.7.0",
     "shiki": "^1.29.2",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
@@ -40,6 +37,7 @@
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
+    "shadcn": "^4.7.0",
     "tailwindcss": "^4.3.0",
     "typescript": "^5.5.0",
     "vite": "^5.3.4"