docs: adopt standard community files

[itsreverence]

Summary

• replace the catch-all docs/WORKFLOW.md with standard contributor-facing CONTRIBUTING.md, focused docs/TESTING.md, and maintainer-only docs/RELEASING.md
• add GitHub-recognized support and pull-request guidance
• keep the README user-facing and remove the duplicated static release version
• update agent and test-harness references to the new canonical docs

docs/ARCHITECTURE.md remains because it is concise, code-tied, and still describes real runtime boundaries rather than duplicating contributor instructions.

Verification

• uv run ruff check . — passed
• uv run pytest -q — 81 passed
• git diff --check — passed
• local Markdown links — no missing targets
• stale docs/WORKFLOW.md references — none