feat(infra): publish CDK constructs to Construct Hub

**Context:** ROADMAP.md → CDK constructs library

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## Component

CDK / infrastructure

## Describe the feature

Publish reusable ABCA constructs to **Construct Hub** with **semver** versioning for external adopters embedding Task API, Blueprint, or FanOut patterns.

## Use case

Forkers duplicate CDK code instead of consuming a versioned library. AWS samples consumers want npm package + Construct Hub discovery.

## Proposed solution

1. Extract publishable constructs package (`@abca/constructs` or similar).
2. jsii pipeline, semver, changelog.
3. Construct Hub registration + docs examples.
4. Keep internal monorepo on workspace link until stable API.
5. Document adoption path in developer guide.

## Other information

- Pairs with **exposed project specifications** draft.
- Design context: `docs/design/ARCHITECTURE.md`.

- [x] This might be a breaking change

Read ROADMAP.md at the CDK constructs library entry and docs/design/ARCHITECTURE.md first. Map the proposed publishable package, jsii and semver pipeline, Construct Hub registration, examples, workspace-link transition, and developer-guide adoption path. Done means these design and delivery pieces are defined well enough to publish and document the reusable constructs.

Written by the indexing model from the issue text.