Epic: #1976

Remove 5 items marked #[allow(dead_code)] that have no production callers:

1. FOCUS_REMINDER_PREFIX — crates/zeph-core/src/agent/focus.rs:75
2. FocusState::should_remind() — focus.rs:126
3. ToolRateLimiter::is_tripped() — rate_limiter.rs:302
4. CorrectionKind::Abandonment — feedback_detector.rs:69
5. SidequestState::parse_eviction_response() — sidequest.rs:178

Acceptance Criteria:

• Zero #[allow(dead_code)] in non-test code
• All tests pass

Complexity: Trivial | Effort: 30m