Split Worker route handlers and support modules

[dills122]

Summary

• Splits the Worker god file into focused modules by routing, handlers, storage, security helpers, and Durable Object responsibilities.
• Keeps behavior unchanged while making the OAuth/session/security code easier to review and maintain.

What Changed

• Reduced apps/worker/src/index.ts to a small Worker router.
• Moved Durable Object behavior into auth-coordinator.ts.
• Split OAuth, session/logout, settings, account deletion, and starred repository proxy logic into dedicated handler modules.
• Extracted shared Worker concerns into http, env, crypto, session-store, settings, rate-limit, github, coordinator-client, constants, and types modules.
• Moved Worker test fixtures and mocks into test/helpers.mjs.

Validation

• pnpm --filter @forage/worker test
• pnpm --filter @forage/worker typecheck
• pnpm --filter @forage/worker build
• pnpm lint
• pnpm check

Scope Notes

• This is a behavior-preserving refactor.
• No auth, storage, rate-limit, or API behavior was intentionally changed.
• DB and analysis-package refactors are intentionally left for separate branches.