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/coder/web/vite.config.d.ts

@@ -1,2 +0,0 @@
-declare const _default: import("vite").UserConfig;
-export default _default;

apps/coder/web/vite.config.js

@@ -1,25 +0,0 @@
-import { defineConfig } from 'vite';
-import react from '@vitejs/plugin-react';
-import path from 'node:path';
-export default defineConfig({
-    plugins: [react()],
-    resolve: {
-        alias: {
-            '@': path.resolve(__dirname, './src'),
-        },
-    },
-    server: {
-        port: 5174,
-        proxy: {
-            '/api': {
-                target: 'http://127.0.0.1:3000',
-                changeOrigin: true,
-                ws: true,
-            },
-        },
-    },
-    build: {
-        outDir: 'dist',
-        emptyOutDir: true,
-    },
-});