feat: redesign todo tool with structured API and key-based operations

[avoidwork]

Description

Full redesign of the todo tool to make it agent-friendly. Replaces unstructured string responses with structured JSON, numeric IDs with semantic string keys, and adds missing actions (delete, list) with proper input validation.

Type of Change

• Breaking change (API: string responses → structured JSON, id → key, create array → single item)
• Bugfix
• New feature
• Documentation update
• Refactor
• Performance improvement
• CI / build / tooling

Testing

• 24 unit tests covering all 7 actions (read, create, update, complete, delete, list, clear)
• Input validation tests for all required fields
• Duplicate key rejection, not-found errors, filter behavior, maxTodos enforcement
• All tests pass, lint passes, formatting passes

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied

Implementation

src/tools/todo.js — Full rewrite

• Structured JSON responses — Every action returns { ok: true/false, ... } with errors in an error field
• String keys — Replaced numeric id with semantic key (e.g., "fix-login-bug")
• New actions — Added delete (remove single todo) and list (with pending/completed filter)
• Input validation — Every action validates required fields before I/O via validateRequired()
• Simplified create — Single { key, content, completed? } instead of array
• Runtime options — filePath and maxTodos configurable via factory

tests/unit/tool_todos.test.js — Full rewrite

• 24 tests covering all actions, validation, edge cases
• Uses configurable temp directory for isolation

Archived

OpenSpec change fix-todo-tool archived to openspec/changes/archive/2026-06-10-fix-todo-tool/. Delta spec synced to openspec/specs/todo-tool/spec.md.