feat: add PR intent labeling workflow

[aaronpowell]

Summary

• add a PR intent labeling workflow for contribution and automation categories
• suppress content labels when a PR appears to be branched from main with materialized plugin files
• add a dedicated external-plugin label when plugins/external.json changes

Testing

• npm run build
• npm run plugin:validate
• npm run skill:validate
• bash eng/fix-line-endings.sh

[Copilot]

Pull request overview

Adds an automated GitHub Actions workflow to apply and maintain a set of “intent” labels on PRs based on touched files (and a couple of heuristics), helping triage contributions and automation-related changes.

Changes:

• Introduces .github/workflows/label-pr-intent.yml to auto-apply/remove a curated set of labels based on PR file paths.
• Adds heuristics to (a) flag PRs targeting main, (b) suppress content labels when plugin materialized artifacts appear present, and (c) apply a dedicated external-plugin label when plugins/external.json changes.

Show a summary per file

File	Description
.github/workflows/label-pr-intent.yml	New pull_request_target workflow that creates/maintains a set of managed labels and applies them based on changed files.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2