chore: prepare reviewer routing and label definitions for taxonomy migration #5502

User Story

As an OpenShell maintainer preparing the label migration in #5151, I need nested SDK subscription topics to work with reviewer routing and a reviewable label-definition plan so I can establish the agreed taxonomy without changing any issue or PR label assignments.

Description

Implement the repository-side prerequisites for #5151. Support the agreed topic:sdk:go, topic:sdk:python, topic:sdk:rust, and topic:sdk:typescript topics independently of topic:sdk. Copy the existing broad SDK reviewers into each language section; individual subscriptions will be refined separately. Remove the obsolete L7 exclusion from the networking scope.

Keep the intended active and retired label definitions, read-only planner/tests, and rollout runbook only on purp/label-migration-tools in purp/OpenShell. These tools are not merged upstream. The draft #5503 must contain only permanent routing/roster/guidance changes. The fork-only branch planner compares every page of live definitions with the trusted roster. The output must preserve label identity for retirement, record original definitions for recovery, and identify creation/update/retirement operations without applying them.

Example preview: uv run --no-project python tasks/scripts/prepare_label_taxonomy.py --repo NVIDIA/OpenShell.

Context

Part of #5151. Live label-definition creation, recoloring, and retirement remain in that preparation issue after routing support lands and external automation compatibility is established. No item-level relabeling or repository label mutations are included here.

Definition of Done

• Nested SDK topics parse and route independently of the broad SDK topic; malformed names remain excluded.
• The roster contains the four language scopes seeded with copies of the existing broad SDK reviewers, and networking no longer excludes L7.
• A complete definition catalog and read-only preview cover the intended active and retired names, preserve source definitions, and reject conflicts or roster drift.
• Focused routing and planner tests pass; contributor guidance explains safe rollout and the remaining external-automation prerequisite.