docs(specs): rewrite aw-harness.md in W3C specification format

- Add W3C-style title block, Abstract, Status of This Document
- Add RFC 2119 Conformance section with keyword table
- Add formal Terminology and Definitions section for all key terms
- Restructure Architecture as normative section with numbered Design Principles
- Add Harness Invocation Contract (entry point, env vars, exit codes, streams)
- Restructure Workflow Definition with formal sub-schemas for each harness.* key
- Add normative Overrides and Fixed Settings table (Section 6.2)
- Add formal DAG Execution Model with normative algorithm (Section 7)
- Restructure all 8 extensions with Requirements + non-normative examples
- Move Model Resolution to its own normative section (Section 9)
- Move Build/Deployment and Implementation Plan to informative section (Section 10)
- Preserve full backwards-compatibility table including gh-proxy override rows
- Add Security Considerations and Privacy Considerations sections
- Add Normative and Informative References section
- Fix duplicate Design Principle number (two items numbered 5)
- Fix env var inconsistency: use AWF_API_PROXY_TOKEN consistently throughout
- Make gh-proxy/cli-proxy always-on constraint a MUST NOT normative requirement
- Frame all code examples as non-normative with >[!NOTE] callouts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot