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/api/client.ts

@@ -11,7 +11,6 @@ import type {
   AgentsResponse,
   GitMeta,
   Skill,
-  AskUserAnswer,
   ToolCostStat,
   ProviderSnapshotEntry,
   CoderProvidersFile,
@@ -284,17 +283,6 @@ export const api = {
         method: 'POST',
         body: JSON.stringify({ skill_name: skillName, user_message: userMessage }),
       }),
-    // Batch 9.7: submit answers for a paused ask_user_input call. Server
-    // validates against the question shape, UPDATEs the pending tool row,
-    // publishes the deferred tool_result frame, and enqueues the next turn.
-    answerUserInput: (chatId: string, toolCallId: string, answers: AskUserAnswer[]) =>
-      request<{ tool_message_id: string; assistant_message_id: string }>(
-        `/api/chats/${chatId}/answer_user_input`,
-        {
-          method: 'POST',
-          body: JSON.stringify({ tool_call_id: toolCallId, answers }),
-        },
-      ),
     // v1.13.17-cross-repo-reads: resume a paused request_read_access. On
     // 'allow' the server re-resolves the grant root and appends it to
     // sessions.allowed_read_paths; the returned list reflects the post-