feat: add send_file built-in tool to send files from sandbox to Telegram

[chinkan]

Summary

• Adds a send_file built-in tool allowing the AI agent to send files from the sandbox to the Telegram user
• Follows the existing tool-based pattern — agent calls an explicit tool when it wants to send a file
• Sandbox path validation reused via validate_sandbox_path()
• File size checked before reading (50 MB Telegram limit)
• Friendly status message: "📤 Sending a file"

What Changed

File	Change
src/tools.rs	Made validate_sandbox_path pub; added send_file ToolDefinition
src/agent.rs	Added send_file execution arm with Telegram send_document API call, ChatId plumbing
src/platform/tool_notifier.rs	Added "📤 Sending a file" friendly name

Test Plan

• cargo build --release — builds clean
• cargo clippy -- -D warnings — no warnings
• cargo fmt --check — formatting clean
• cargo test — 378 tests pass