fix(skills): Add missing standalone PR fallback for fresh feature branches

The decision tree in create-java-pr Step 0 had a gap: when a non-main
branch has no existing PR and no downstream PRs target it, no outcome
was specified. This is the most common case (fresh feature branch).

Add explicit fallback to standalone PR context, matching the behavior
of the parallel branch where a PR exists with base main and no
downstream PRs.

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

Author: adinauer

.claude/skills/create-java-pr/SKILL.md

@@ -40,7 +40,8 @@ git branch --show-current
     ```bash
     gh pr list --base "$(git branch --show-current)" --json number,headRefName,title
     ```
-    If there are downstream PRs, treat this as **next PR in an existing stack** with the current branch as the stack base (collection branch).
+    - If there are downstream PRs, treat this as **next PR in an existing stack** with the current branch as the stack base (collection branch).
+    - If there are no downstream PRs either, treat it as **standalone PR context** (fresh feature branch).
 
 3. If signals are mixed or ambiguous, ask one focused question to confirm.