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.

apps/server/src/services/read_tab_by_number.ts

@@ -3,7 +3,7 @@
 // stored in the session's workspace_panes envelope (WorkspaceState.tabNumbers),
 // keyed by chat id. Lives in its own file (not appended to tools.ts) so tests
 // can import the executor directly without dragging in the whole tool registry.
-// Registered in tools.ts ALL_TOOLS + READ_ONLY_TOOL_NAMES.
+// Registered in tools.ts ALL_TOOLS.
 
 import { z } from 'zod';
 import type { Sql } from '../db.js';