Fix packaged skill resource reads

[FredKSchott]

Summary

• expose packaged skill supporting files through a dedicated read_skill_resource framework tool
• stop synthesizing a packaged-only tool named read for sandbox adapters that intentionally replace default filesystem tools
• advertise the dedicated resource tool during progressive disclosure and reserve its framework-owned name

Why

Sandbox adapter tools replace Flue's default model-facing filesystem tools. When an adapter omitted read, registering a packaged skill caused Flue to inject a packaged-only tool named read. Models could reasonably pass workspace paths to it and receive a misleading Packaged skill file not found error even though the workspace file existed.

A dedicated tool matches the Agent Skills client guidance: activate instructions first, then load bundled supporting resources on demand without implying general sandbox filesystem authority.

Verification

• pnpm run test in packages/runtime (759 tests)
• pnpm run check:types in packages/runtime
• Biome check for all changed files