Docs: publish OpenAPI/JSON Schema and integration index

**Context:** ROADMAP.md → Exposed project specifications

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## Doc area

Design / architecture (`docs/design/`)

## Describe the issue

ABCA lacks published **machine-readable API specs** and a single **integration/extension index**. Contributors and operators must reverse-engineer handlers and design docs to extend or fork the platform.

## Affected docs

- `docs/design/API_CONTRACT.md`
- `docs/guides/DEVELOPER_GUIDE.md`
- New: generated OpenAPI/JSON Schema artifact (location TBD, e.g. `docs/spec/` or release asset)
- Starlight site API reference page (after `mise //docs:sync`)

## Suggested change

1. Generate OpenAPI 3.x from REST handlers or maintain hand-authored spec with CI drift check.
2. Publish JSON Schema for key request/response types (sync with `cdk/src/handlers/shared/types.ts` / `cli/src/types.ts`).
3. Add **extension-point index**: webhooks, channels, workflows, Blueprint hooks, Cedar actions.
4. Stable permalinks from spec to architecture contracts listed at end of former ROADMAP.md.
5. CI gate: spec matches implementation (similar to types-sync check).

## Other information

- Complements **CDK constructs library** draft for external consumers.
- Does not replace narrative design docs.

Start with docs/design/API_CONTRACT.md, docs/guides/DEVELOPER_GUIDE.md, and the types in cdk/src/handlers/shared/types.ts and cli/src/types.ts; review the REST handlers and the existing types-sync check. The work is done when published OpenAPI and JSON Schema artifacts, an extension-point index, stable architecture links, and a CI drift check are available, with the Starlight API reference updated after mise //docs:sync.

Written by the indexing model from the issue text.