feat(governance): expose stable machine-readable GovernanceDecision contract (#29)

[ElliotSun]

Summary

Exposes a versioned, machine-readable public representation for SemaPact governance decisions (PublicGovernanceDecisionV1), establishing a stable, deterministic integration contract for CI/CD gates, CLI output, PR annotations, APIs, and future external consumers without leaking internal domain types.

Key features:

• PublicGovernanceDecisionV1 Model (semapact/governance/public.py): Immutable, strict, versioned schema (schemaVersion: "1") with camelCase JSON serialization and snake_case Python attribute support via AliasChoices.
• Public Protocol Vocabulary: Explicit Literal definitions for public vocabulary (PublicDecisionResult, PublicRequiredVersionBump, PublicSeverity, PublicChangeType, PublicEntityType, PublicChangeDomain, PublicEvidenceSource) without speculative additions.
• Strict One-Way Boundary: Domain layer remains decoupled from presentation/public projections (GovernanceDecision has zero dependencies on public.py).
• Deterministic Canonical Serialization: serialize_public_governance_decision() provides byte-level deterministic JSON with sort_keys=True and sorted reasonCodes / evidence.
• Read-Only Golden Fixtures: Added 5 golden JSON fixtures in tests/fixtures/governance_decisions/ verified by strict string equality tests.

Closes #29

Architecture

Owned by semapact.governance.public layer. Adheres to the strict unidirectional flow:
Domain GovernanceDecision $\longrightarrow$ Projection Layer $\longrightarrow$ PublicGovernanceDecisionV1.

Validation

• Tests added or updated where behavior changed (tests/test_public_governance_decision.py)
• uv run pytest (445 passed)
• uvx ruff check . --select E9,F63,F7,F82 (All checks passed)
• uv build (Built sdist and wheel successfully)

Governance / compatibility impact

Introduces the stable public schema version 1 for governance decisions. Does not alter internal evaluation logic or break existing CLI / pipeline behavior.