fix(skill): Detect stacked PR context from branch (#5223)

* fix(skill): Detect stacked PR context from branch

Update the create-java-pr skill to infer standalone vs stacked PR mode\nfrom git branch and existing PR relationships.\n\nWhen running on main/master, default to standalone PR mode and only\nenter stack mode when explicitly requested by the user.\n\nCo-Authored-By: Claude <noreply@anthropic.com>

* fix(create-java-pr): Detect collection branches with downstream PRs

Check for downstream PRs even when the current branch already has a PR targeting main/master.

This prevents collection branches in a stacked PR flow from being misclassified as standalone PR context.

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

* 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>

* 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>

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: adinauer

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

@@ -9,15 +9,48 @@ Prepare local changes and create a pull request for the sentry-java repo.
 
 **Required reading:** Before proceeding, read `.cursor/rules/pr.mdc` for the full PR and stacked PR workflow details. That file is the source of truth for PR conventions, stack comment format, branch naming, and merge strategy.
 
-## Step 0: Determine PR Type
+## Step 0: Determine PR Type From Git Branch Context
 
-Ask the user (or infer from context) whether this is:
+Infer PR type from the current branch before asking the user.
 
-- **Standalone PR** — a regular PR targeting `main`. Follow Steps 1–6 as written.
-- **First PR of a new stack** — ask for a topic name (e.g. "Global Attributes"). Create a collection branch from `main`, then branch the first PR off it. The first PR targets the collection branch.
-- **Next PR in an existing stack** — identify the previous stack branch and topic. This PR targets the previous stack branch.
+1. Get current branch:
 
-If the user mentions "stack", "stacked PR", or provides a topic name with a number (e.g. `[Topic 2]`), treat it as a stacked PR. See `.cursor/rules/pr.mdc` § "Stacked PRs" for full details.
+```bash
+git branch --show-current
+```
+
+2. Apply these rules:
+
+- **If branch is `main` or `master`**: default to a **standalone PR**.
+  - Do **not** assume stack mode from `main`.
+  - Only use stack mode if the user explicitly asks for a stacked PR.
+- **If branch is not `main`/`master`**:
+  - Check whether that branch already has a PR and what its base is:
+    ```bash
+    gh pr list --head "$(git branch --show-current)" --json number,baseRefName,title --jq '.[0]'
+    ```
+  - If that branch PR exists and `baseRefName` is **not** `main`/`master`, treat the work as a **stacked PR context**.
+  - If that branch PR exists and `baseRefName` **is** `main`/`master`, also check whether other PRs target the current branch:
+    ```bash
+    gh pr list --base "$(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 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 **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.
+
+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 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.
 
 ## Step 1: Ensure Feature Branch