docs: adopt Agent Decision Comments

[dbrattli]

Summary

• adopt Agent Decision Comments specification 0.1.1 as a repository-local convention
• add repository-wide contributor and agent guidance
• document durable scheduling, lifecycle, supervision, buffering, backpressure, and cross-target constraints near the code they govern
• annotate relevant test and CI infrastructure decisions

Impact

This is a documentation-only change. It preserves non-obvious engineering rationale for human reviewers and future coding agents without changing executable behavior.

Validation

• dotnet fantomas --check src test extra/AsyncSeq
• just build
• dotnet build test
• dotnet run --project test --no-build — 87/87 passed
• Fable compilation for JavaScript, Python, and BEAM
• YAML syntax validation
• git diff --check

Note

The plain just test invocation currently fails inside dotnet run's implicit build with no compiler diagnostics. Building the test project explicitly and running with --no-build succeeds with all 87 tests passing.