[Spec] Finalize v1.6.0 and Transition to Stable Release

[tercel]

Problem Description

Protocol v1.6.0 is currently in draft status in PROTOCOL_SPEC.md. While the content is stable and recent features like the Config Bus Architecture and Error Formatter Registry have been added, it lacks a formal Stable designation.

Why solve it

A stable specification is critical for:

1. Foundation Submission: Signaling maturity to AAIF/CNCF during the project acceptance process (planned for 2026 Q2).
2. Ecosystem Stability: Providing a reliable foundation for SDKs (Python, TS, Rust) and adapters (MCP, A2A, CLI).
3. Adoption: Encouraging enterprise and community adoption by guaranteeing specification stability.

How to solve it

1. Audit: Review all TODO, MAY, and draft notes in PROTOCOL_SPEC.md.
2. Verification: Ensure all v1.6.0 features are implemented and verified in at least two reference SDKs (Python and TypeScript).
3. Status Update: Update the Status field in PROTOCOL_SPEC.md to Stable and remove the -draft suffix from the version.
4. Documentation: Synchronize all docs/ files to ensure they reference the stable v1.6.0 semantics.