Ignore .claude/ in .gitignore (closes #204)

REPO_POLICIES.md says in-repo agent scratch belongs in both .gitignore and
.dockerignore. .dockerignore already has .claude; .gitignore now has the
.claude/ entry and its comment exactly as the canonical .gitignore in
sneak/prompts has them, unanchored so it matches at every depth.

Model: opus-5-5

.gitignore

@@ -11,6 +11,12 @@ Thumbs.db
 .vscode/
 *.sublime-*
 
+# Agent scratch (worktrees of this repo, created and destroyed by
+# in-flight tooling). Unanchored: .gitignore patterns already match at
+# every depth, so no prefix is wanted here. This is not a .dockerignore
+# entry and must not be given a `**/` prefix on the way into one.
+.claude/
+
 # Environment / secrets
 .env
 .env.*