ci: remove pull_request_target workflow

Removes the semantic PR title lint workflow that ran on pull_request_target.
This trigger grants secrets/write tokens to workflows triggered from forks,
which is risky. Drop the workflow rather than rewire it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/semantic-pr-lint.yml

@@ -1,19 +0,0 @@
-name: semantic-pr-lint
-
-on:
-  pull_request_target:
-    types:
-      - opened
-      - edited
-      - reopened
-
-jobs:
-  main:
-    name: Validate PR title
-    runs-on: ubuntu-latest
-    permissions:
-      pull-requests: read
-    steps:
-      - uses: amannn/action-semantic-pull-request@48f256284bd46cdaab1048c3721360e808335d50 # v6
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}