chore: migrate area and topic labels to the new taxonomy #5151

User Story

As an OpenShell maintainer, I need unresolved issues and unmerged PRs to use the agreed area and topic taxonomy so that current and potentially reopened work can be found and routed to the right reviewers.

Description

After #4863 lands, inventory the current area:* and topic:* labels and migrate applicable issues and PRs to the agreed new taxonomy. Develop and review an initial mapping, then revise it using the actual contents of affected items. This issue is for planning and carrying out the follow-up; the mapping below is a starting point for discussion, not an approved bulk transformation.

Include items regardless of open or closed state. Leave merged PRs and resolved issues unchanged where possible. An issue is resolved if any of the following applies:

• It is closed as completed.
• It has an attached merged PR that closed it.
• Its author commented that they are closing it, including abandonment or displacement by another issue.
• A maintainer commented that the project is not accepting the work.

Review closure comments and maintainer status in context; do not infer resolution solely from a closed state or silently guess when evidence is ambiguous. Record uncertain cases for human disposition. Treat PRs as merged based on their merge status, including merged PRs later reverted.

Initial mapping for discussion

These are candidate classifications based on the scopes in #4863; one old label may map to multiple new labels depending on the item. Preserve any existing intended labels and unrelated labels. Do not blindly add every suggested category to every item.

Create missing intended labels, including topic:extensions, with reviewed descriptions and colors. Determine which old labels should be retired and how to prevent future use. Deleting a GitHub label removes it from historical items too, so preserve labels on resolved issues and merged PRs where possible; explicitly decide how to handle labels that cannot be globally removed without changing that history.

Context

• Depends on the final taxonomy in feat(review): add area and topic reviewer rosters and routing #4863 landing; reconcile the mapping with the merged version before executing.
• Reviewer-routing automation may react to label changes on ready PRs. Assess and plan for notifications and review requests during migration.
• Include a dry run or reviewable inventory showing proposed additions and removals, ambiguous cases, and skipped resolved/merged items. Keep a record sufficient to audit and reverse changes if needed. Recheck item status before applying changes to account for concurrent updates.

Definition of Done

• Inventory the existing and intended taxonomy, discuss and revise the mapping, and identify ambiguous cases for human review.
• Establish the intended labels and prevent further use of retired labels without inadvertently stripping labels from historical items.
• Identify eligible issues and PRs across open and closed states using the resolution rules above, and document any exceptions.
• Review the proposed changes and account for reviewer-routing side effects before applying them.
• Migrate eligible items, preserve unrelated labels, and retain a reviewable change record and recovery path.
• Verify coverage, skipped items, and any remaining exceptions; report the outcome.