feat: Migrate issue type/priority to native fields; drop milestones

[joeyparrish]

Replace the 'type: ...' and 'priority: Px' label scheme with GitHub's native issue Type and Priority fields, and remove the now-unused milestone maintenance from the update-issues tool.

update-issues:

• Read the native issue type (from the REST issues API) instead of 'type: ...' labels. Question pings key on the native 'Question' type.
• Remove milestone maintenance entirely (milestones are no longer used), along with the Milestone class, version parsing, and the priority reading that only existed to route milestones. This also drops the tool's need for any GraphQL field access.
• Explicit workflow permissions (issues/pull-requests: write) were already declared; only stale milestone wording was updated.

sync-labels:

• Remove all 'type: ...' and 'priority: Px' label definitions (they are now native fields) and update the README.
• Transition 'type: external' to 'flag: external'.

Other:

• Remove the archived triage-party-config repo from the policy and common-workflow sync lists and delete its label config.
• policies/SECURITY.md: drop the 'type: vulnerability' label reference.

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[joeyparrish]

Note that merging this PR will trigger updates to synced workflows across several repos.