toolkit merge: Phase B — extract @sentry/toolkit-core (shared code)

[BYK]

Sub-task of the toolkit monorepo merge (master issue linked below). Plan: https://gist.github.com/BYK/8c0ce6cec64608f41fe5654b7827228e

Phase B (fast-follow): extract @sentry/toolkit-core. Incrementally move
shared surfaces behind a workspace dep, one seam at a time, with existing
property/model-based tests as the safety net:

• Sentry API client + auth/OAuth device flow + token storage.
• Region detection / multi-region fan-out.
• Shared Zod types (issues/events/projects/orgs).
• Telemetry setup + error taxonomy.
• Agent-skill generation / plugin-marketplace machinery.

This is where the long-term ROI of the merge is realized. See §7.

[BYK]

Master tracking issue: #1248