Create `preparing-pr` skill (#9752)

* Create `preparing-pr` skill

* use relative path

.agents/skills/preparing-pr/SKILL.md

diff --git a/.agents/skills/preparing-pr/SKILL.md b/.agents/skills/preparing-pr/SKILL.md
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+---
+name: preparing-pr
+description: Prepare a pull request by running presubmit with fixes. Use when preparing to commit or send a PR.
+---
+
+# Preparing a Pull Request
+
+Follow these steps to prepare a pull request for DevTools:
+
+1.  **Verify `dt` configuration**:
+    -   Ensure `dt` is available on the path by running `dt --help` or verifying its alias.
+    -   If not available, refer to the [CONTRIBUTING guide](../../../CONTRIBUTING.md#L64-L80) for instructions on how to set up the `dt` executable on your path.
+2.  **Update Flutter SDK**:
+    -   Run `dt update-flutter-sdk` to ensure the SDK is up to date.
+3.  **Run Presubmit with Fixes**:
+    -   Run `dt presubmit --fix` to address automated fixes.
+    -   If any issues remain or if the command fails, fix the issues manually and run `dt presubmit --fix` again to verify the fix.
+4.  **Run Tests for Affected Code**:
+    -   Use `git status` or `git diff upstream/master` to find changed files.
+    -   Run tests that are associated with the changed code. For example, if you edited `packages/devtools_app/lib/src/screens/logging/logging_screen.dart`, run the logging tests under `packages/devtools_app/test/screens/logging/`.
+
+## Verification
+
+If any step fails, stop and address the issue, and then verify the issue is fixed before proceeding.
+
+> [!IMPORTANT]
+> Do NOT commit or push changes without getting explicit user approval first.