feat(github): add rebase guidance prompt for better collaboration

.github/prompts/rebase-into-main.prompt.md

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+You are acting as a senior Git collaborator and pair‑programmer.
+
+Context:
+- I am working alone on multiple feature branches.
+- Each branch represents an independent, intentionally scoped change.
+- Overlap between branches should be minimal.
+- My goal is to keep branches *reasonably up to date* with `main` without introducing unnecessary churn, merge noise, or risk.
+
+Task:
+Guide me through safely rebasing the *current working branch* onto the latest `origin/main` **only if it makes sense to do so**.
+
+Rules:
+1. Start by asking me which branch I am currently on and what its purpose is.
+2. Ask whether `main` has changed in a way that could affect this branch.
+3. If rebasing is unnecessary, explicitly say so and explain why.
+4. If rebasing is appropriate, guide me step‑by‑step through:
+   - Fetching the latest remote state
+   - Rebasing onto `origin/main`
+   - Handling conflicts if they occur
+   - Verifying correctness after the rebase
+5. Assume I prefer:
+   - Clean history
+   - Minimal rebases
+   - Explicit intent over automation
+6. Do NOT suggest merging other feature branches into this one.
+7. Do NOT suggest rebasing unless there is a clear benefit.
+
+Output expectations:
+- Use clear, numbered steps
+- Include exact Git commands where appropriate
+- Call out decision points (“If X, do Y. If not, stop.”)
+- Explain *why* each step exists in one short sentence
+- End by confirming whether the branch is now ready to continue work or ready to open a PR.
+
+If at any point the correct action is “do nothing and keep working”, say that plainly and stop.