Split tools.js and engine.js into per-domain modules per GUIDELINES.md

[neooriginal]

This issue is AI generated.

Problem

Two files dramatically exceed the ~300 line limit set in GUIDELINES.md and create real maintenance, review, and security problems:

• server/services/ai/tools.js — ~2800 lines, one massive switch statement. Every tool's security properties, SSRF risk, side-effect classification, and data access must be reasoned about by reading a single giant file. Adding a tool, reviewing a tool's security posture, or changing behaviour for one category requires reading past thousands of unrelated lines.

• server/services/ai/engine.js — ~3400 lines. Task analysis, context building, history compaction, multi-step execution, parallel sub-agent orchestration, memory consolidation, and skill learning all live in one class. GUIDELINES.md: "If a file grows past ~300 lines, split it."

Required change

tools.js — Group tool handler functions by domain into separate modules:

• server/services/ai/tools/browser.js
• server/services/ai/tools/shell.js
• server/services/ai/tools/memory.js
• server/services/ai/tools/files.js
• server/services/ai/tools/messaging.js
• server/services/ai/tools/http.js
• etc.

A thin tools/index.js routes the toolName to the correct module.

engine.js — Extract cohesive concerns into collaborating modules:

• server/services/ai/runner.js — the main loop and iteration logic
• server/services/ai/planner.js — task analysis and plan construction
• server/services/ai/executor.js — tool dispatch and result handling

Public API of engine.js remains unchanged; it re-exports from the new modules.

Acceptance criteria

• No file in server/services/ai/ exceeds 500 lines.
• All existing tests pass without modification.
• Each new module is independently testable.
• No circular imports are introduced.