fix: add PR merge conflict check to dev agent workflow

Dev agent should check if PR has merge conflicts before asking for review.
This prevents approving a PR that has conflicts, which could introduce
new errors after rebase.

Changes:
- Add conflict check step in workflow for both with/without user message cases
- Use mergeable field from PR API to detect conflicts
- Instructions to rebase and resolve conflicts if mergeable: false

skills/kugetsu/scripts/kugetsu-session.sh

@@ -280,6 +280,11 @@ Workflow:
 1. Read the issue at $instance/$owner/$repo/issues/$number AND all comments on that issue
 2. Check if a PR already exists for this issue
    - If PR exists and is open, review it and learn from it
+   - CRITICAL: Check if PR has merge conflicts before asking for review:
+     - Use: curl -s "https://$instance/api/v1/repos/$owner/$repo/pulls/$number" -H "Authorization: Bearer \$GITEA_TOKEN"
+     - If "mergeable": false, there ARE conflicts - you MUST resolve them FIRST
+     - To resolve: cd to worktree, git fetch origin, git rebase origin/main, resolve conflicts, git rebase --continue, git push --force-with-lease
+     - Only after resolving conflicts (mergeable: true) can you ask for review
    - If PR makes sense to continue, work on it instead
    - If PR is not worth continuing, create a new branch/PR but explain in PR description why you're creating a new one instead of continuing the existing PR
 3. Read README.md (if exists) to understand the general concept of this repository
@@ -314,6 +319,11 @@ Workflow:
 1. Read the issue at $instance/$owner/$repo/issues/$number AND all comments on that issue
 2. Check if a PR already exists for this issue
    - If PR exists and is open, review it and learn from it
+   - CRITICAL: Check if PR has merge conflicts before asking for review:
+     - Use: curl -s "https://$instance/api/v1/repos/$owner/$repo/pulls/$number" -H "Authorization: Bearer \$GITEA_TOKEN"
+     - If "mergeable": false, there ARE conflicts - you MUST resolve them FIRST
+     - To resolve: cd to worktree, git fetch origin, git rebase origin/main, resolve conflicts, git rebase --continue, git push --force-with-lease
+     - Only after resolving conflicts (mergeable: true) can you ask for review
    - If PR makes sense to continue, work on it instead
    - If PR is not worth continuing, create a new branch/PR but explain in PR description why you're creating a new one instead of continuing the existing PR
 3. Read README.md (if exists) to understand the general concept of this repository