[codex] Add current document context command

[afar1]

Summary

Adds ft current, a CLI command that reads the active Field Theory document context written by the Mac app terminal integration.

The command finds the newest context manifest under the Field Theory Library, reads the pointed-at active.md, and can print either human-readable output, JSON, or content-only markdown.

Why

Agents in the embedded terminal need a stable way to answer “what document am I looking at?” without relying on manual find/jq commands or prompt-only context.

Changes

• Add src/current.ts for context manifest discovery, validation, formatting, and content loading.
• Register ft current, ft current --json, ft current --content-only, and --manifest <path>.
• Teach the Field Theory skill text to use ft current --json for active document questions.
• Add tests for manifest selection and command registration.

Validation

• npm ci
• npx tsx --test tests/current.test.ts tests/cli.test.ts tests/skill.test.ts
• npm run build
• npm run dev -- current --json
• git diff --check
• npm test (545 passed)