docs(adr): compatibility rule for mixed versions during a rollout #178

🤖 AI agent: ADR needed for the rolling agent rollout (#176); refs #168, #174.

Filed from the owner account at the owner's request in chat; the bot identity was not available in this session.

Problem

During a rolling rollout, agent pods of the previous version run next to the new router, Claude proxy, egress proxies, API and operator. Worker and forge sidecar are in one pod, so they always change together, but nothing today states or tests that the platform accepts the previous worker version.

Change

1. ADR (docs/adr/): the compatibility rule. Proposed: the platform services agent pods call (router wake, Claude proxy, forge sidecar to API, approvals) accept the previous MINOR version; CRD changes are additive (new fields optional, no removed or renamed field within one MINOR step); a breaking internal change needs two releases (accept both, then drop the old). States what counts as internal interface (not part of the SemVer public interface in CLAUDE.md section 7, but covered by this rule).
2. CI test: on kind, run the previous release's worker and forge images against the platform of the PR head and do the e2e pass (issue, PR, review, reply).
3. CLAUDE.md section 7 and docs/concepts.md link the ADR; reviewer checklist gets a line for it if the owner agrees.

Acceptance criterion

The ADR is merged; the CI job runs on every PR and passes on main; one deliberately incompatible change in a test branch makes it fail.

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