refactor: centralize agentic frontend event projection

[limityan]

Summary

• Move AgenticEvent frontend event name/type/payload projection into bitfun-events as a framework-neutral contract.
• Simplify Tauri and WebSocket transport adapters so they only deliver projected events instead of owning event field mapping.
• Preserve the legacy WebSocket agentic event allowlist and dialog-turn-started wire shape, with focused regression tests and boundary rules.
• Update architecture/plan docs to record the stable event-projection owner without adding process notes.

Risk and compatibility

• No intended product behavior change.
• Tauri event names and payload fields stay centralized in the shared projection.
• WebSocket keeps the previous limited event set and does not add userInput to dialog-turn-started.
• bitfun-events remains free of Tauri, React, and transport adapter dependencies.

Verification

• cargo test -p bitfun-events
• cargo test -p bitfun-transport
• cargo check -p bitfun-transport --features tauri-adapter
• cargo check -p bitfun-core --no-default-features
• cargo check --workspace
• cargo metadata --no-deps --format-version 1
• node --test scripts/check-core-boundaries.test.mjs
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• git diff --check

Note: local cargo check --workspace required an ignored empty src/mobile-web/dist directory because the desktop Tauri build script checks that path.