Support built-in tool selection for agent profiles

[astrobot-houston]

Originally proposed by @uriafranko in #259. Their description is reproduced verbatim below.

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Summary

• add optional builtInTools config for agent profiles and runtime configs
• preserve the current full built-in tool set when builtInTools is omitted
• allow disabling all built-ins or selecting specific built-in tool names
• document the new option and export related authoring types from @flue/runtime

Validation

• pnpm --filter @flue/runtime test -- tool.test.ts agent-definition.test.ts
• pnpm --filter @flue/runtime check:types
• pnpm --filter @flue/runtime build
• pnpm --filter apps-docs check

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Original implementation from #259 by @uriafranko

[uriafranko]

The idea here is to allow deployment on more than coding agent and open this harness to other usecases beyond find related.