chore: add task spec for issue 258

.core/task/issue/258/spec.md

@@ -0,0 +1,29 @@
+# Issue 258: Smart Test Detection
+
+## Original Issue
+https://github.com/host-uk/core/issues/258
+
+## Summary
+Make `core test` smart - detect changed files and run only relevant tests.
+
+## Commands
+```bash
+core test                    # Run tests for changed files only
+core test --all              # Run all tests  
+core test --filter UserTest  # Run specific test
+core test --coverage         # With coverage report
+```
+
+## Acceptance Criteria
+- [ ] Detect changed `.go` files via `git diff --name-only`
+- [ ] Map source files to test files (`foo.go` → `foo_test.go`)
+- [ ] Run only relevant tests via `go test`
+- [ ] Support `--all` flag to run all tests
+- [ ] Support `--filter` flag for pattern matching
+- [ ] Support `--coverage` flag for coverage reports
+
+## Technical Context
+- Go CLI using Cobra
+- Commands in `cmd/core/cmd/`
+- Follow existing patterns in `dev_*.go` files
+