Write Stage 5 release diagnostics summary

[anth-volk]

Stage 5 PR 5.7 should add release_diagnostics_summary.json as a compact run-scoped release summary.

Scope:

• join high-level Stage 2, Stage 3, Stage 4, and Stage 5 facts from existing structured run artifacts when present
• represent missing upstream diagnostics explicitly instead of inventing new Stage 2 or Stage 3 schemas
• write the summary during release promotion and reference it from release_promotion_contract.json
• include the summary in the Stage 5 run diagnostics/step manifest surface for dashboard and AI discovery
• add tests for missing upstream diagnostics, partial summaries, and complete summaries

This tracks the Stage 5 release promotion refactor lane step pr-5.7.