refactor(tools): move GetToolSpec presentation contracts

[limityan]

Summary

• Move pure GetToolSpec presentation contracts into bitfun-agent-tools: prompt description, input schema, validation, assistant-detail rendering, collapsed-tool catalog entry formatting, and duplicate-load hint text.
• Keep runtime ownership unchanged in bitfun-core: GetToolSpecTool, context-aware collapsed-tool discovery, ToolUseContext.unlocked_collapsed_tools, manifest resolver, registry snapshot, and concrete tools still stay in core.
• Extend owner-crate contract tests, strengthen scripts/check-core-boundaries.mjs, and update the decomposition docs plus core AGENTS guidance to reflect the boundary.

Safety / behavior boundary

• No tool execution path, runtime manifest assembly, unlock state, concrete tool implementation, or feature behavior is moved.
• Core only delegates stateless rendering/schema/validation helpers to the owner crate, preserving the existing prompt text, schema shape, duplicate-load result, and assistant XML-section escaping behavior.
• Rebased against GCWing/main (b7a289b0) before opening the PR.

Validation

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