fix(core): omit unavailable host tools

[kitlangton]

Summary

• add an additive OpenCode host configuration for unavailable tools
• remove unavailable built-in and application tool definitions before prompting
• settle stale calls to unavailable tools as terminal unknown-tool errors
• prevent headless hosts from creating unanswered question interactions

Verification

• application tools, Session runner, public OpenCode, and location layer suites
• Core, Server, and OpenCode application typechecks
• git diff --check origin/dev...HEAD

[kitlangton]

Closing for API redesign. A special layerWithHostConfig({ unavailableTools }) constructor is the wrong abstraction; tool visibility should follow the existing permission/catalog model. We will replace this with a smaller permission-based design.