Author contract-ledger.md taxonomy documentation

Context Discussion #10703 finalized the "Contract Completeness Gate", which requires a formalized Contract Ledger matrix for public-facing API changes.

The Problem There is currently no documentation defining what the Contract Ledger is, what the L1-L4 taxonomy for contracts entails, and what the structural shape of the matrix must be.

The Fix Create learn/agentos/contract-ledger.md (sibling to evidence-ladder.md) documenting the L1-L4 taxonomy and the exact Contract Ledger matrix schema required.

Acceptance Criteria

• learn/agentos/contract-ledger.md is authored.
• L1-L4 taxonomy is defined.
• Matrix schema (Target Surface, Source of Authority, Proposed Behavior, Fallback, Docs, Evidence) is documented with examples.

Related

• Parent Epic: #10704
• Discussion #10703