RFC: ToolHive Core Shared Library with Package Graduation Criteria

Proposes creating toolhive-core, a shared Go library with explicit API
stability guarantees for the ToolHive ecosystem. Establishes formal
graduation criteria for promoting packages from internal to shared status.

Key points:
- Graduation criteria: stability, quality, dependencies, governance
- Tier 1 packages: logger, errors, validation, healthcheck, versions, env
- Based on actual exploration of toolhive, toolhive-registry-server, dockyard
- Go 1.25+ minimum version

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: JAORMX