refactor(agent-tools): extract tool manifest contracts

[limityan]

Summary

• Move pure tool manifest/exposure contracts into bitfun-agent-tools: ToolExposure, GetToolSpec name, manifest policy resolution, collapsed prompt stub construction, and prompt-visible ordering.
• Keep runtime behavior in core: product registry snapshot, context-aware schema/description, GetToolSpecTool execution, ToolUseContext, unlock state, snapshot decoration, and concrete tool implementations remain core-owned.
• Update boundary checks and decomposition docs so this contract closure is not mistaken for runtime owner migration.
• Add focused owner-crate contract tests, including manifest definition serde round-trip and existing GetToolSpec insertion/ordering behavior.

Guardrails

This PR is intended to be behavior-preserving. It does not change product tool availability, expanded/collapsed exposure semantics, prompt-visible collapsed stubs, unlock behavior, desktop/MCP/ACP catalog behavior, or concrete tool execution paths.

Validation

• cargo test -p bitfun-agent-tools -- --nocapture
• cargo test -p bitfun-core manifest_resolver -- --nocapture
• cargo test -p bitfun-core get_tool_spec -- --nocapture
• cargo check -p bitfun-core --features product-full
• node scripts/check-core-boundaries.mjs
• git diff --check