Ignore the local design symlink and agent instruction files (#17)

Adds /design, /AGENTS.md and /CLAUDE.md to .gitignore. None of the three
are tracked: `design` is a symlink to the penguin-harness-design
checkout and `CLAUDE.md` is a symlink to `AGENTS.md`, so until now they
only showed up as untracked noise in `git status`.

.gitignore

@@ -29,3 +29,8 @@ cache/
 # playwright e2e artifacts
 packages/web/test-results/
 packages/web/playwright-report/
+
+# local design-doc symlink and agent instructions
+/design
+/AGENTS.md
+/CLAUDE.md