chore: automate pull request labels

[nahime0]

Summary

Add automated pull request classification and labeling for elephc.

The automation:

• infers type:* from the PR title, falling back to the branch prefix;
• selects up to three primary area:* labels from changed files;
• detects target-specific changes through target:*;
• assigns size:* and scope:multi-area when appropriate;
• preserves manually assigned topic:* and priority labels;
• creates missing labels from the declarative catalog automatically.

A manual workflow dispatch supports individual PR numbers or all-open, with dry_run: true by default for safely previewing a
backfill.

Safety

The workflow uses pull_request_target, but:

• checks out only the trusted base commit;
• never checks out or executes code from the PR head;
• disables persisted checkout credentials;
• uses explicit, limited GitHub token permissions.