refactor(agent-tools): prepare tool provider ownership

[limityan]

Summary

• Adds an agent-tools static-provider installation contract and moves core builtin tool assembly into stable core-owned provider groups.
• Adds portable tool context facts plus a read-only PortableToolContextProvider projection from core ToolUseContext.
• Completes the low-risk tool-packs feature scaffold metadata for planned tool groups without registering or moving concrete tools.
• Updates decomposition guardrails, AGENTS notes, and boundary checks so product tool runtime ownership remains explicit.

Functional Boundary

This PR is behavior-preserving. Concrete tool implementations, ToolUseContext, runtime manifest assembly, GetToolSpec execution, collapsed-tool unlock state, snapshot decoration, and desktop/MCP/ACP tool catalog behavior remain core-owned.

The new contracts are scaffolding for later owner migration only; they do not change tool availability, tool execution, feature defaults, product runtime assembly, or platform behavior.

Validation

• cargo test -p bitfun-agent-tools -- --nocapture
• cargo test -p bitfun-tool-packs --features product-full -- --nocapture
• cargo test -p bitfun-core path_resolution_tests --lib -- --nocapture
• cargo test -p bitfun-core registry_ --lib -- --nocapture
• cargo test -p bitfun-core manifest_ --lib -- --nocapture
• cargo test -p bitfun-core get_tool_spec --lib -- --nocapture
• node scripts/check-core-boundaries.mjs
• cargo check -p bitfun-core --features product-full
• cargo tree -p bitfun-agent-tools --depth 1 --edges features
• cargo tree -p bitfun-tool-packs --features product-full --depth 1 --edges features
• git diff --check