feat: add view_image tool

[wgqqqqq]

Summary

• add a built-in view_image tool that attaches local or remote-workspace image bytes as multimodal tool output
• expose the tool to multimodal-capable agents and register it in the product tool pack
• support local absolute paths, local workspace paths, and remote SSH workspace-relative paths

Verification

• cargo test -p bitfun-core view_image_ -- --nocapture
• cargo test -p bitfun-core product_tool_materializer_preserves_provider_plan_order -- --nocapture
• cargo test -p bitfun-tool-packs product_provider_group_plan_preserves_builtin_tool_order
• cargo check -p bitfun-core
• cargo check -p bitfun-core --no-default-features
• git diff --check