feat: 3lang depth PropGraph unification - phase refinements, wiring, tests + session pickup cleanups/governance
- Complete depth canonical, pack resolver DEPTH_PACKS, pipeline attrs+node_depths+graph_anti, contemplate/pass_manager/contracts propagation (immutable).
- Polish anti_unifier, tests (oov, construction, percent), runtime/chat integration.
- Governance: AGENTS.md updates, docs/README/handoff_template, skills bootstrap, retire handoff.
- Leaves pickup seeds (NEW_SESSION_PROMPT, plan.md, compact) for continuity.
- Spine tests green, 3lang depth visible in ctx for he.
Refs: plan.md, 2026-07-06-compact.md
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