feat(tools): add send_image tool for channel image delivery

[maximilize]

Summary

• Add send_image tool that lets skills send local image files (PNG, JPEG, GIF, WebP) to channel targets like Telegram
• Reuses the existing screenshot pipeline — returns a data: URI in the screenshot key, which the chat runner picks up automatically
• Supports optional caption parameter sent as the media message text
• 20 MB file size guard (Telegram limit)
• Fix send_screenshot_to_channels to parse actual MIME type from the data URI instead of hardcoding image/png

Validation

Completed

• cargo check passes
• cargo test — all tests pass (7 new unit tests for SendImageTool)
• cargo +nightly-2025-11-30 fmt --all -- --check passes
• cargo +nightly-2025-11-30 clippy --workspace --all-targets -- -D warnings passes

Remaining

• ./scripts/local-validate.sh (requires CI environment)

Manual QA

1. Create a skill that saves an image to /tmp/test.png
2. Call send_image with path: "/tmp/test.png"
3. Verify Telegram receives the image
4. Repeat with .jpg, .gif, .webp to confirm MIME handling
5. Test with caption parameter — verify caption appears in Telegram
6. Test with file > 20 MB — verify rejection
7. Test with unsupported extension (.bmp) — verify rejection

[penso]

Thanks, rewrote because of conflicts in #224