Refine compact System App UI and add file deletion

[siwei-yuan]

Summary

• wrap compact keyboard input and bound chat previews to their real card height
• add a Files delete mode matching the Apps uninstall UI
• route Files UI and the Agent-facing workspace.delete Tool through one Pi Agent deleteFile Action
• mount declared App filesystems in Action Guests and publish file-backed View invalidation through the existing App revision
• update System App ownership, source contract, and runtime documentation

Architecture

The Pi Agent System App owns deleteFile. UI and Agent actors invoke the same Action. Native code only provides generic capability-based filesystem mounts, bounded Action execution, and revision delivery. The View SDK callback is generic and carries no App data.

Validation

• cargo test --workspace: 55 passed
• cargo clippy --workspace --all-targets -- -D warnings
• bun test apps/pi-agent/text.test.js: 3 passed
• ESP32-S3 release firmware build passed
• Simulator visual checks passed at 480x800 and 720x1280
• ESP32-S3 physical E2E passed for compact text layout, Files delete UI, deletion, and list refresh