Show test coverage from the cached `.coverage_baseline` file, or rerun the full test suite with `$ARGUMENTS`.

## Usage

- `/coverage` — read cached coverage from `.coverage_baseline` (instant)
- `/coverage run` — run `script/test_coverage` and report fresh results

## What it does

**Cached mode (default):** Reads `.coverage_baseline` and displays the stored coverage percentage(s). This is instant and does not run any tests.

**Run mode (`run`):** Executes `script/test_coverage` which runs:
1. Rust tests with `cargo llvm-cov` (reports line coverage %)
2. Frontend tests with `npm run test:coverage` (reports line coverage %)
3. Computes the overall average and compares to the threshold

Reports Rust coverage, Frontend coverage, Overall coverage, and whether the run passed the threshold.

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If the arguments (`$ARGUMENTS`) equal `run`, execute `bash script/test_coverage` from the project root and show the Coverage Summary section from the output. Otherwise, read `.coverage_baseline` and display the stored coverage value(s).
