Epic: #1975

Every LLM provider independently maintains last_usage: Mutex<Option<(u64, u64)>> and last_cache: Mutex<Option<(u64, u64)>> with identical logic. 60 lines duplicated across 4 providers.

Extract UsageTracker struct in zeph-llm/src/usage.rs:

pub(crate) struct UsageTracker {
    last_usage: Mutex<Option<(u64, u64)>>,
    last_cache: Mutex<Option<(u64, u64)>>,
}

Locations: claude/mod.rs:54-55, openai/mod.rs:26-27, ollama.rs:35, gemini/mod.rs:42

Acceptance Criteria:

• UsageTracker used by all 4 providers
• Clone implementation resets to None
• All provider tests pass

Complexity: Small | Effort: 2h
Ref: DRY-01