[Quality audit] Simplify MCP coordinator and tool surface

[johannesjo]

Derived from the local audit report CODE_QUALITY_FINDINGS.md created on 2026-05-30.

Scope

This issue tracks complexity and duplication in electron/mcp/**.

Findings

• Finding 9: tested MCP config builder is bypassed.
  • Evidence: buildCoordinatorMCPConfig is defined and tested, but StartMCPServer manually rebuilds the same object.
  • Fix direction: call buildCoordinatorMCPConfig in StartMCPServer.
• Finding 10: sub-task MCP config construction is duplicated.
  • Evidence: duplicated mcpServers["parallel-code"] construction in restart rewrite, task creation, and hydration.
  • Fix direction: extract buildSubTaskMcpConfig, writeSubTaskMcpConfig, and isAllowedSubTaskMcpConfigPath; confirm mcpServerInfo.token contract.
• Finding 15: agent output monitoring is copied in MCP coordinator.
  • Evidence: duplicated base64 decode, tail buffering, ANSI stripping, prompt detection, idle transitions, and resolver flushing.
  • Fix direction: extract createAgentOutputMonitor(task) and handlePromptDetected(task).
• Finding 16: task teardown cleanup repeats in MCP coordinator.
  • Evidence: removeCoordinatedTask, cleanupTask, and deregisterCoordinator repeat lifecycle cleanup with subtle behavior differences.
  • Fix direction: extract narrow helpers for subscriptions, idle waiters, config cleanup, and control-state clearing.
• Finding 17: MCP createTask has too many responsibilities.
  • Evidence: electron/mcp/coordinator.ts:398-740.
  • Fix direction: move preamble write/restore logic into preamble.ts and use small queued file mutation helpers.
• Finding 18: MCP tool diff formatting and truncation is duplicated.
  • Evidence: electron/mcp/server.ts and electron/mcp/coordinator.ts both format/truncate diffs.
  • Fix direction: centralize formatDiffForTool(result, mergeInfo?) and one MAX_DIFF_BYTES.
• Finding 19: deprecated review_and_merge_task is still advertised.
  • Evidence: electron/mcp/mcp-tool-list.ts, electron/mcp/server.ts, electron/mcp/client.ts.
  • Fix direction: keep compatibility handler if needed but remove the tool from advertised tools.
• Finding 35: validation has an unreachable specific branch and duplicate tests.
  • Evidence: electron/mcp/validation.ts, duplicated validation coverage in coordinator tests.
  • Fix direction: move the @{ check before the generic invalid-character regex, or remove the branch; keep validation tests in validation.test.ts.

Acceptance checks

• MCP coordinator test suite
• MCP validation tests
• MCP tool-list tests
• token-rotation, hydration, Docker path, prompt, idle, and cleanup cases