docs: extract TESTING.md and compress CHANGELOG (#400)

- New TESTING.md gathers unit / selftest / E2E commands, NativeAOT
  publish steps, and the unit+selftest coverage workflow that were
  previously duplicated in CONTRIBUTING.md.
- CONTRIBUTING.md keeps a short quick-reference and links to
  TESTING.md.
- AGENTS.md gains a one-line pointer to TESTING.md.
- CHANGELOG.md compressed 1380 -> 179 lines: one paragraph per
  significant initiative (specs 033 / 034 / 036 / 038 / 039 / 042 /
  045), per-task implementation detail rolled up. Contributor
  conventions now call out "focus on significant/breaking changes"
  explicitly.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris

AGENTS.md

@@ -29,6 +29,8 @@ dotnet test tests/Reactor.AppTests --filter "ClassName=Reactor.AppTests.Tests.Ac
 
 CI runs unit tests + selftests + full solution build on every PR. .NET 10 SDK, `windows-latest` runner.
 
+Full testing guide — tier selection, NativeAOT runs, code coverage — in [`TESTING.md`](TESTING.md).
+
 ## Architecture
 
 ### Virtual DOM model