Document FableCodex script and state architecture

[baskduf]

Description

Add an architecture note for contributors now that the script/state structure is more mature after v0.4.4.

Suggested file: docs/ARCHITECTURE.md

Acceptance Criteria

• Document the plugin layout.
• Document wrapper scripts and their target Python scripts.
• Document .codex-fable5/ local state files:
  • goals.json
  • findings.json
  • ledger.jsonl
  • state.lock
• Document the relationship between goals, findings, and the shared codex_fable_state.py helpers.
• Document the high-level CI/release validation flow.
• Link the architecture doc from README.md or CONTRIBUTING.md.

Notes

Keep this concise and practical for new contributors.