Detect and guard against stale watch/index-writer version skew

[amazon7737]

Context

A previously observed operational failure allowed a resident kleos watch process from an older binary to rewrite index.json in an older shape, causing newer PR reverse-lookup fields to disappear. The current recovery is a manual daemon restart after upgrading.

Acceptance criteria

• Define a backward-compatible writer compatibility signal for the resident watch/index path.
• Detect an incompatible or stale index writer before newer fields can disappear silently, and emit an actionable English message.
• Add a synthetic regression test that models an old-shape rewrite after a newer index has been produced.
• Preserve schema-v1 read compatibility and existing incremental indexing behavior.
• Document the detected condition and safe recovery path.
• Do not terminate user processes automatically or rewrite private archive content destructively.
• Pass targeted race tests and the repository-wide validation suite.

First safe step

Write a synthetic compatibility test that reproduces the field-loss sequence without touching a real archive or daemon.

Non-goals

• Process termination, deployment, or service-manager integration.
• Publishing archive data or inspecting real user sessions.