doc: Finalize CONTRACT_onboarding_action.md (examples + state/action table)

[Whiteks1]

Context

The onboarding API contract must be the single source of truth for frontend ↔ backend onboarding interactions. It must be explicit and verifiable (real JSON, real states, real action values).

Affected paths

• docs/ai/api/CONTRACT_onboarding_action.md

Steps

1. Add a clear State Machine section:
   • list canonical states for onboarding_basic: start, ask_name, ask_goal, summary, end
   • allowed transitions and what triggers them
2. Add an Actions section with the real enum used by the client (e.g. START, SUBMIT_TEXT, SELECT_OPTION) and explain:
   • when each action is valid
   • what value contains for each action
3. Add request/response JSON examples per state (valid JSON):
   • happy path
   • invalid action
   • invalid/empty value
4. Add explicit rule: no extra questions in summary and end.
5. Add a small “Error responses” section (shape + examples).
6. Ensure this contract does not silently define states that are not in onboarding_basic (design-only flows must be clearly labeled as out-of-scope for this Epic).

Definition of Done

• Contract includes a state table and action table
• All JSON examples are valid JSON and match the described schema
• Canonical states for onboarding_basic are explicitly documented
• Summary/end rule (“no extra questions”) is explicit
• No contradictions with docs/ai/workflows/onboarding_basic.yaml and docs/ai/tests/onboarding_basic.md