Add fail-closed journal diagnostics

[VeryComplexAndLongName]

Adds structured compatibility diagnostics for persisted run journals while preserving fail-closed recovery behavior.

• distinguishes invalid JSON, invalid shape, future journal/checkpoint versions, invalid checkpoints, and workspace mismatches
• preserves incompatible journal bytes and original path without quarantine or rewrite
• returns actionable upgrade guidance through standalone recovery endpoints
• displays the same core guidance from the VS Code extension
• records the persistence policy in ADR 0006

Verification:

• npm run verify (268 tests)
• npm run build
• openspec change validate --strict journal-format-diagnostics
• live authenticated future-journal smoke with HTTP 409 and unchanged SHA-256