ci(plugin): drop with: from the plugin job's checkout

Run 3027: the Plugin hooks job failed at checkout, before the script ran.
Adding a `with: fetch-depth: 0` block made actions/checkout@v6 fail to
extract on the act_runner —

  Cannot find module '/var/run/act/actions/<sha>/dist/index.js'

— while every bare `uses: actions/checkout@v6` in the same run succeeded.
The runner's action-cache handling is the difference, not git.

No depth was needed in the first place. The version check compares two
TREES, and a tree diff needs both trees, not a common ancestor. Verified
against a real depth-1 clone: after `git fetch --depth=1 origin
main:refs/remotes/origin/main`, both `git diff origin/main -- plugin` and
`git show origin/main:plugin/.claude-plugin/plugin.json` work. So the
explicit fetch already in the step is sufficient, and cheaper than the
full history the `with:` block was asking for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

scripts/check_plugin.py

@@ -171,8 +171,9 @@ def check_version_bump(base: str = "origin/main") -> None:
         # of bug survives in the first place.
         fail(
             f"cannot resolve {base}, so the version-bump check could not run. "
-            f"Fetch it (checkout needs fetch-depth: 0) or pass --no-version "
-            f"deliberately."
+            f"Fetch it first — `git fetch --depth=1 origin main:refs/remotes/"
+            f"origin/main` is enough, since this diffs two trees and needs no "
+            f"common ancestor — or pass --no-version deliberately."
         )
         return