[V1][individual] Email-to-Todo Converter - Core feature engine

[Banx17]

Overview

This PR adds the core feature engine for the Email-to-Todo Converter tool inside its isolated folder. The implementation follows the established tool pattern (matching follow-up-reminder and grammar-cleaner) with a layered services/guards/ui architecture.

Related Issue

Closes #355

Changes

Core Engine (services/):

• [ADD] services/emailToTodo.ts — Pure deterministic conversion logic with types, constants, and functions: buildTaskDraft, detectPriority, suggestDueDate, buildTaskTitle, buildTaskNotes, hasConvertibleContent
• [ADD] services/guards.ts — Security/validation guard layer: sanitizeText, validateEmailInput, sanitizeEmailInput, checkInputLimits, safeBuildTaskDraft entry point
• [ADD] services/fixtures.ts — 7 deterministic synthetic fixtures covering direct request, urgent, newsletter, empty subject, blank content, medium priority, and label scenarios

UI Refactor (ui/):

• [MODIFY] ui/emailToTodoView.ts — Stripped to view-model helpers only (describeConverter, resolveStatusMessage); imports types from services
• [MODIFY] ui/EmailToTodoConverter.tsx — Updated to use safeBuildTaskDraft from guard layer
• [MODIFY] ui/index.ts — Updated exports to re-export from services

Tests & Config:

• [ADD] tests/emailToTodo.test.ts — 27 engine and view-model tests
• [ADD] tests/guards.test.ts — 33 guard, fixture, and integration tests
• [DEL] tests/emailToTodoView.test.ts — Replaced by new test files
• [ADD] vitest.config.ts — Local vitest configuration

Docs:

• [MODIFY] README.md — Updated with architecture overview
• [MODIFY] REVIEW_NOTES.md — Documented all changes

Verification Results

npm test (vitest):
✅ 61/61 tests passed (2 test files, 0 failures)

All work stays within tools/v1/individual/email-to-todo-converter/. No main app, routing, inbox, wallet, database, or design-system integration.

Acceptance Criteria

Status	Criterion
✅	Core logic is implemented without linking into the main app
✅	Inputs, outputs, loading states, and error states are documented
✅	No live network calls, secrets, or production data are introduced
✅	Files changed are limited to tools/v1/individual/email-to-todo-converter/
✅	The contribution is reviewable as a self-contained mini-product change

[Banx17]

I've pushed a formatting fix for the Client Checks. CI needs an approval to re-run. Could a maintainer please approve the workflow run?