proof: add graph-ready relationship envelopes and policy lineage

[davidahmann]

Problem

PolicyGraph-ready relationships and lineage metadata needed to be emitted consistently from day one across core artifacts (intent/trace/run/session/scout), while preserving additive compatibility and deterministic behavior. We also needed to align Gait with released proof v0.4.5.

Changes

• Bumped github.com/Clyra-AI/proof to v0.4.5 in go.mod/go.sum.
• Added a standard optional relationship envelope in core artifact contracts and runtime emission paths:
  • gate intent/trace/audit records
  • run/session timeline + checkpoint/session events
  • scout inventory records
• Added optional policy lineage fields beyond digest where available:
  • policy_id, policy_version, matched_rule_ids
• Added normalization helpers for relationship envelopes and wired lineage propagation through MCP/voice/session paths.
• Extended schema + fixture coverage for additive compatibility and invalid-shape rejection.
• Added deterministic contract tests for with/without relationship fields and unknown-field tolerance.
• Updated contract docs and enterprise consumer contract script to reflect relationship-first envelope expectations.

Validation

• make prepush-full
• push hook make prepush (ran automatically on git push)

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