story-kit: create 120_story_test_coverage_llm_chat_rs

.story_kit/work/1_upcoming/120_story_test_coverage_llm_chat_rs.md

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+---
+name: "Add test coverage for llm/chat.rs (2.6% -> 60%+)"
+---
+
+# Story 120: Add test coverage for llm/chat.rs
+
+Currently at 2.6% line coverage (343 lines, 334 missed). This is the chat completion orchestration layer — the biggest uncovered module by missed line count.
+
+## What to test
+
+- Message construction and formatting
+- Token counting/estimation logic
+- Chat session management
+- Error handling paths (provider errors, timeout, malformed responses)
+- Any pure functions that don't require a live LLM connection
+
+## Notes
+
+- Mock the LLM provider trait/interface rather than making real API calls
+- Focus on the logic layer, not the provider integration
+- Target 60%+ line coverage
+
+## Acceptance Criteria
+
+- [ ] Line coverage for `llm/chat.rs` reaches 60%+
+- [ ] Tests pass with `cargo test`
+- [ ] `cargo clippy` clean