docs(httpapi): update migration spec to current state

[kitlangton]

Summary

Docs-only update to packages/opencode/specs/effect/http-api.md so it reflects what's actually shipped:

• Rewrite Current State to describe backend selection at server startup (server/backend.ts + server.ts), replacing the earlier in-Hono path-mounting bridge model.
• Note that auth is centrally configured for the Effect backend via Effect `Config` (cee9610d2 refactor: use Effect config for HttpApi authorization, 3250b814c Fix HttpApi raw route authorization) rather than re-attached per route module.
• Mark Phase 4 (OpenAPI/SDK generation from Effect) as implemented and opt-in (bun dev generate --httpapi, OPENCODE_SDK_OPENAPI=httpapi), and document the schema-shape gaps in the Effect-generated spec that block flipping the SDK generator default:
  • Branded-type pattern constraints not propagated (~169 missing).
  • Per-property description annotations not propagated (~107 missing).
  • Event.* / SyncEvent.* component-name dotted vs PascalCase divergence.
  • Component-deduper numbered duplicates (Session9, SyncEvent.session.updated.11).
• Update Remaining PR Plan and bottom checklist:
  • could you add ollama and OLLAMA_BASE_URL support? #11 (event SSE) clarified — Effect backend uses raw Effect HTTP; Hono backend still uses hono/streaming.
  • status messages and logging need work #12 / Enhance custom commands #13 (pty / tui) reframed as "Effect backend done; Hono backend files still present".
  • feature: interactive setup wizard #14 reframed as opt-in implemented, gap-closing pending.
  • panic: status service not initialized. Call status.InitService() at application startup. when running opencode #15 reframed around backend.ts default flip rather than per-route flag.

Test plan

• N/A — documentation only. Verify by reading the rendered diff.