docs(agent): consolidate AGENT.md as canonical, remove copilot-instructions.md

Make AGENT.md the single canonical source of truth for AI agents working in this repo, and remove the now-redundant .github/copilot-instructions.md.

Changes to AGENT.md:

- Merge in all unique content from .github/copilot-instructions.md: Key Features list, Client Factory method names, full DTO + REST/Search code examples, Coding Guidelines numbered list, Documentation Guidelines, Handling int64 Values, example test structure block.

- Fix stale jest references throughout (the repo uses vitest per package.json).

- Update single-test commands to vitest syntax with correct env-file sourcing for unit and integration tests.

- Update sm:start/stop scripts list to match current package.json (8.8 and 8.9 stack helpers).

- Add @camunda8/orchestration-cluster-api to the Dependencies list with a pointer to MAINTAINER.md for the 8.x pinning rationale.

- Header now explicitly states AGENT.md is canonical and supersedes copilot-instructions.md.

- Cross-references to CONTRIBUTING.md and MAINTAINER.md preserved.

Stacks on top of #758 (commit-conventions section) by re-asserting the same content here. If #758 lands first, this PR will rebase cleanly to a no-op for that section.

Author: jwulf