fix(create-java-pr): Map stack base detection to defined PR type

When downstream PRs are found for a branch, classify the result as an existing stack flow instead of an undefined "stack base context".

Clarify that the next PR in an existing stack can target either the previous stack PR branch or the collection branch, so all detection outcomes map to actionable PR types.

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

Author: adinauer

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

@@ -34,20 +34,20 @@ git branch --show-current
     ```bash
     gh pr list --base "$(git branch --show-current)" --json number,headRefName,title
     ```
-    - If there are downstream PRs, treat this as a **stack base context** (collection branch).
+    - If there are downstream PRs, treat this as **next PR in an existing stack** with the current branch as the stack base (collection branch).
     - If there are no downstream PRs, treat it as **standalone PR context**.
   - If no PR exists for the current branch, check whether other PRs target it:
     ```bash
     gh pr list --base "$(git branch --show-current)" --json number,headRefName,title
     ```
-    If there are downstream PRs, treat this as a stack base context.
+    If there are downstream PRs, treat this as **next PR in an existing stack** with the current branch as the stack base (collection branch).
 
 3. If signals are mixed or ambiguous, ask one focused question to confirm.
 
 PR types:
 - **Standalone PR** — regular PR targeting `main`.
 - **First PR of a new stack** — create collection branch from `main`, then first PR off it.
-- **Next PR in an existing stack** — target previous stack branch.
+- **Next PR in an existing stack** — target the current stack base branch (usually the previous stack PR branch, or the collection branch if creating the first follow-up PR from the collection branch).
 
 If the user explicitly says "stack", "stacked PR", or provides numbered stack titles (e.g. `[Topic 2]`), honor that even if branch heuristics are inconclusive.