task(ceremony): contract as explicit traceability discipline in implement and verify

[pentaxis93]

Summary

Part of #191. Contract defines behavior traceability as a lifecycle discipline (specify → implement → verify → land), but the protocol procedures in implement and verify don't enforce it. The behavior-contract artifact flows through these protocols without procedural requirements to maintain traceability.

Scope

File	Change
protocols/implement/PROTOCOL.md	Add behavior traceability requirement to implement-behavior procedure
protocols/verify/PROTOCOL.md	Add behavior-indexed evidence step to gate function

Acceptance Criteria

• implement implement-behavior procedure requires naming which contract behavior(s) each RED test advances — an agent cannot write a test without tracing it to the contract
• verify gate function requires behavior-indexed evidence: for each claimed behavior, cite the verifying test; report behavior coverage alongside command output
• Cross-references updated where missing
• Consistent with contract's discipline statement: "If you cannot name which behavior a code change advances, the change is unanchored"

Dependencies

None — independent of the other tasks in #191.

Evidence

• implement/PROTOCOL.md lines 208-218: implement-behavior has no step requiring behavior traceability — agent can write RED tests without referencing the contract
• verify/PROTOCOL.md lines 34-48: gate function treats evidence as command output, not behavior coverage
• skills/contract/SKILL.md lines 31-34: "If you cannot name which behavior a code change advances, the change is unanchored"
• skills/contract/SKILL.md lines 40-41: "'All tests pass' is incomplete if you cannot name which claimed behaviors those tests verify"
• orient/SKILL.md line 66: "Behavior traceability — the behavior contract from stage 2 should be traceable at every subsequent stage"