Follow-up from PR #1696 security audit (SEC-COMPACT-03).

Currently, if Anthropic deprecates or rejects the compact-2026-01-12 beta header, all Claude requests with server_compaction=true will return a 400 error with no fallback. The client should:

1. Detect a 400/invalid beta header response
2. Automatically disable server_compaction_active for the session
3. Fall back to client-side compaction
4. Log a warning so the user knows to update their config

Files: crates/zeph-llm/src/claude.rs, crates/zeph-core/src/agent/