docs: add test reliability and Graphite CLI notes to CLAUDE.md (#270)

## Summary
- Add test reliability policy: all test failures must be treated as real
regressions, no skipping or workarounds
- Add note to prefer `gt` (Graphite CLI) over `gh` for creating PRs when
available

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

CLAUDE.md

@@ -36,6 +36,8 @@ just lint-windows  # Cross-clippy for Windows (requires cargo-xwin)
 just doc           # Documentation generation
 ```
 
+If `gt` (Graphite CLI) is available in PATH, use it instead of `gh` to create pull requests.
+
 ## Tests
 
 ```bash
@@ -48,6 +50,10 @@ INSTA_UPDATE=always cargo test                          # Update snapshots
 
 Integration tests (e2e, plan, fspy) require `pnpm install` in `packages/tools` first. You don't need `pnpm install` in test fixture directories.
 
+### Test Reliability
+
+The test suite has no known pre-existing failures or flaky tests. If a test fails during your changes, treat it as a real regression caused by your work. Fix the root cause properly — do not skip, ignore, or work around failing tests.
+
 ### Test Fixtures
 
 - **Plan**: `crates/vite_task_plan/tests/plan_snapshots/fixtures/` — quicker, sufficient for testing task graph, resolved configs, program paths, cwd, and env vars