chore: add implementation plan for issue 258

.core/task/issue/258/plan.md

@@ -0,0 +1,33 @@
+# Implementation Plan: Issue 258
+
+## Phase 1: Command Structure
+1. Create `cmd/core/cmd/test.go`
+2. Register `test` command with Cobra
+3. Add flags: `--all`, `--filter`, `--coverage`
+
+## Phase 2: Change Detection  
+1. Run `git diff --name-only HEAD` to get changed files
+2. Filter for `.go` files (exclude `_test.go`)
+3. Map each file to its test file:
+   - `internal/foo/bar.go` → `internal/foo/bar_test.go`
+   - Skip if test file does not exist
+
+## Phase 3: Test Execution
+1. Build `go test` command with detected test files
+2. Pass through `--coverage` flag as `-cover`
+3. Pass through `--filter` as `-run` pattern
+4. Stream output to terminal
+
+## Phase 4: Edge Cases
+- No changed files → inform user, optionally run all
+- No matching test files → inform user
+- `--all` flag → skip detection, run `go test ./...`
+
+## Files to Create/Modify
+- `cmd/core/cmd/test.go` (new)
+- `cmd/core/cmd/root.go` (register command)
+
+## Testing
+- Add `cmd/core/cmd/test_test.go` with unit tests
+- Manual testing with actual git changes
+