refactor: use typed primary model facts

[wgqqqqq]

Summary

• add PrimaryModelFacts as the typed per-turn primary model snapshot
• pass the snapshot through manifest generation, prompt scaffold, round execution, and ToolUseContext
• remove primary-model facts from tool custom_data legacy projection and migrate view_image/computer_use to typed accessors
• document the design and logging expectations

Verification

• pnpm run fmt:rs
• git diff --check
• cargo check -p bitfun-core
• cargo test -p tool-runtime context::tests -- --nocapture
• cargo test -p bitfun-core tool_context_runtime -- --nocapture
• cargo test -p bitfun-core computer_use -- --nocapture
• cargo test -p bitfun-core view_image -- --nocapture