feat(test): Add smart test detection (issue #258) (#263)

* chore: add task spec for issue 258

* chore: add implementation plan for issue 258

* fix(spec): address CodeRabbit review comments on plan and spec

- Plan: clarify git diff strategy (local HEAD vs CI origin/dev...HEAD)
- Plan: add Phase 2 rename/delete/add handling via git diff --name-status
- Plan: add N:M test file discovery (not just 1:1 mapping)
- Plan: align Phase 3 with existing runTest() infrastructure
- Plan: replace raw `go test ./...` fallback with runTest() call
- Plan: correct file paths to internal/cmd/test/ (not cmd/core/cmd/)
- Spec: explicitly scope as Go-only with note on future language support
- Spec: wrap bare URL in angle brackets
- Spec: add --base flag for CI/PR context
- Spec: update acceptance criteria to match revised plan
- Spec: add technical context pointing to existing infrastructure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* ci: retrigger checks after disabling default CodeQL setup

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -0,0 +1,50 @@
+# Implementation Plan: Issue 258
+
+## Phase 1: Command Structure
+1. Extend existing `internal/cmd/test/cmd_main.go` with smart detection flags
+2. Add flags: `--all`, `--filter` (alias for `--run`)
+3. Existing flags (`--coverage`, `--verbose`, `--short`, `--race`, `--json`, `--pkg`, `--run`) are already registered
+
+## Phase 2: Change Detection
+1. Determine diff strategy based on context:
+   - **Local development** (default): `git diff --name-only HEAD` for uncommitted changes, plus `git diff --name-only --cached` for staged changes
+   - **CI/PR context**: `git diff --name-only origin/dev...HEAD` to compare against base branch
+   - Auto-detect CI via `CI` or `GITHUB_ACTIONS` env vars; allow override via `--base` flag
+2. Filter for `.go` files (exclude `_test.go`)
+3. Use `git diff --name-status` to detect renames (R), adds (A), and deletes (D):
+   - **Renames**: Map tests to the new file path
+   - **Deletes**: Skip deleted source files (do not run orphaned tests)
+   - **New files without tests**: Log a warning
+4. Map each changed file to test file(s) using N:M discovery:
+   - Search for `*_test.go` files in the same package directory (not just `<file>_test.go`)
+   - Handle shared test files that cover multiple source files
+   - `internal/foo/bar.go` → `internal/foo/bar_test.go`, `internal/foo/bar_integration_test.go`, etc.
+   - Skip if no matching test files exist (warn user)
+
+## Phase 3: Test Execution
+1. Reuse existing `runTest()` from `internal/cmd/test/cmd_runner.go`
+   - This preserves environment setup (`MACOSX_DEPLOYMENT_TARGET`), output filtering (linker warnings), coverage parsing, JSON support, and consistent styling
+2. Map smart detection flags to existing `runTest()` parameters:
+   - `--coverage` → `coverage` param (already exists)
+   - `--filter` → `run` param (mapped to `-run`)
+   - Detected test packages → `pkg` param (comma-joined or iterated)
+3. Do not invoke `go test` directly — all execution goes through `runTest()`
+
+## Phase 4: Edge Cases
+- No changed files → inform user, suggest `--all`
+- No matching test files → inform user with list of changed files that lack tests
+- `--all` flag → skip detection, call `runTest()` with `pkg="./..."` (uses existing infrastructure, not raw `go test`)
+- Mixed renames and edits → deduplicate test file list
+- Non-Go files changed → skip silently (only `.go` files trigger detection)
+
+## Files to Modify
+- `internal/cmd/test/cmd_main.go` (add `--all`, `--filter`, `--base` flags)
+- `internal/cmd/test/cmd_runner.go` (add change detection logic before calling existing `runTest()`)
+- `internal/cmd/test/cmd_detect.go` (new — git diff parsing and file-to-test mapping)
+
+## Testing
+- Add `internal/cmd/test/cmd_detect_test.go` with unit tests for:
+  - File-to-test mapping (1:1, 1:N, renames, deletes)
+  - Git diff parsing (`--name-only`, `--name-status`)
+  - CI vs local context detection
+- Manual testing with actual git changes

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

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+# Issue 258: Smart Test Detection
+
+## Original Issue
+<https://github.com/host-uk/core/issues/258>
+
+## Summary
+Make `core test` smart — detect changed Go files and run only relevant tests.
+
+> **Scope:** Go-only. The existing `core test` command (`internal/cmd/test/`) targets Go projects (requires `go.mod`). Future language support (PHP, etc.) would be added as separate detection strategies, but this issue covers Go only.
+
+## Commands
+```bash
+core test                    # Run tests for changed files only
+core test --all              # Run all tests (skip detection)
+core test --filter UserTest  # Run specific test pattern
+core test --coverage         # With coverage report
+core test --base origin/dev  # Compare against specific base branch (CI)
+```
+
+## Acceptance Criteria
+- [ ] Detect changed `.go` files via `git diff` (local: `HEAD`, CI: `origin/dev...HEAD`)
+- [ ] Handle renames, deletes, and new files via `git diff --name-status`
+- [ ] Map source files to test files using N:M discovery (`foo.go` → `foo_test.go`, `foo_integration_test.go`, etc.)
+- [ ] Warn when changed files have no corresponding tests
+- [ ] Execute tests through existing `runTest()` infrastructure (not raw `go test`)
+- [ ] Support `--all` flag to skip detection and run all tests
+- [ ] Support `--filter` flag for test name pattern matching
+- [ ] Support `--coverage` flag for coverage reports
+- [ ] Support `--base` flag for CI/PR diff context
+
+## Technical Context
+- Existing `core test` command: `internal/cmd/test/cmd_main.go`
+- Existing test runner: `internal/cmd/test/cmd_runner.go` (`runTest()`)
+- Output parsing: `internal/cmd/test/cmd_output.go`
+- Command registration: `internal/cmd/test/cmd_commands.go` via `cli.RegisterCommands()`
+- Follow existing patterns in `internal/cmd/test/`