docs: add 2.0 engineering readiness audit

[psimaker]

What & why

Close the final engineering-readiness audit gating the 2.0.0 release train: D021–D025 gap matrix, per-leg evidence states (upload, download, roundtrip, cleanup — never merged), full compatibility matrix, security/privacy/secret and wire review, existing-user/migration/rollback review, helper/relay rollout and downgrade matrices, and the staged app rollout with abort criteria.

Physical-device evidence is recorded as not executed — owner-approved physical-device waiver (2026-07-15) with the simulator and isolated Syncthing substitute evidence. No claim in the audit is stronger than the recorded per-leg evidence; VaultSync 2.0 stays NO-GO until the release-readiness checklist, relay deploy, and staged rollout complete.

Component(s)

• docs only

Testing

• Docs-only change; all referenced evidence was produced and verified at the audited merge heads of feat: add foreground upload-only diagnostics #126, feat: add controlled diagnostics download #127, and feat: derive causal diagnostics roundtrip #128.

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 78fba8ea-19b2-4dd1-ac69-84964c84fab7

📥 Commits

Reviewing files that changed from the base of the PR and between 948b6ec and ca14316.

📒 Files selected for processing (1)

• docs/engineering-readiness-2.0.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/engineering-readiness-audit

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