feat(api): generate synchronized Postman collection

[gbaudrit]

Summary

Adds a generated, synchronized Postman HTTP API collection for Agentstration, backed by the authoritative runtime OpenAPI document and tracked by #284.

Changes

• Adds a .NET generator for a complete 341-operation Postman API reference with separate scheme, host, port, and composed baseUrl variables.
• Adds Local and Docker environments, with credentials and bearer tokens intentionally empty.
• Preserves the Source Registry lifecycle as a curated executable scenario alongside the generated reference.
• Improves OpenAPI tags so requests are grouped by Agentstration domain.
• Adds MSTest contract coverage across registered routes, OpenAPI operations, Postman requests, endpoint variables, duplicate detection, and secret-free environments.
• Adds AGENTS.md, PR checklist, usage documentation, and a CI --check safeguard requiring API changes to regenerate the committed artifacts.

Validation

• CI build and complete test suite: passed (835 total: 831 passed, 4 optional integrations skipped).
• Postman generated-artifact synchronization check: passed.
• PostgreSQL migration/restart test and storage concurrency benchmark: passed.
• Windows host lifecycle: passed.
• Container build: passed.
• Complete AEP SDK build and tests: passed.
• Source Registry tool package smoke test: passed.
• Documentation, Dependency Review, Pull request metadata, and CodeQL workflows: passed.
• JSON parsing and repository diff checks: passed locally.
• No UI behavior changed; desktop and mobile smoke tests are not applicable.

Breaking changes

None.

Closes #284