refactor(core): move tool registry contracts into agent-tools

[limityan]

Summary

• Move lightweight tool contracts plus the generic tool registry / dynamic-provider container into bitfun-agent-tools.
• Keep bitfun-core registry as product runtime assembly: builtin tool list, dyn Tool adapter, snapshot decoration, and legacy compatibility paths.
• Add boundary checks and concise docs/AGENTS guidance so ToolUseContext and concrete tool implementations stay core-owned until a reviewed port/provider design and equivalence tests exist.

Guardrails

• No concrete tool implementation or ToolUseContext migration in this PR.
• No product feature matrix, default feature, build script, or release script changes.
• Dynamic tool metadata, descriptor shape/order, readonly/builtin tool lists, and snapshot wrapping remain covered by tests.

Validation

• node scripts/check-core-boundaries.mjs
• rustfmt --edition 2021 --check src\crates\agent-tools\src\framework.rs src\crates\agent-tools\src\lib.rs src\crates\agent-tools\tests\tool_contracts.rs src\crates\core\src\agentic\tools\framework.rs src\crates\core\src\agentic\tools\registry.rs src\crates\core\src\agentic\tools\restrictions.rs src\crates\core\src\agentic\tools\pipeline\tool_pipeline.rs src\crates\runtime-ports\src\lib.rs
• git diff --check gcwing/main..HEAD
• cargo test -p bitfun-agent-tools
• cargo test -p bitfun-runtime-ports
• cargo test -p bitfun-core registry_ --lib
• cargo test -p bitfun-core dynamic_tool_provider_ --lib
• cargo check --workspace (passes with existing bitfun-cli unused Read warning)
• cargo test --workspace (passes with existing bitfun-cli unused Read warning)