chore: add .gitignore

Excludes build artifacts and the .claude/skills symlink (wired by the skills
installer, never committed — matches the mathias/skills convention) and local
env files.

.gitignore

@@ -0,0 +1,12 @@
+# Build artifacts
+/bin/
+*.out
+
+# Skills are wired as symlinks by the skills library installer and must never
+# be committed (see docs convention in mathias/skills).
+.claude/skills
+
+# Local env / editor
+.env
+.env.*
+*.local