Add CLI file arguments support via @file prefix

Implements ability to include files directly in the initial message using @ prefix.

Features:
- All @file arguments are coalesced into the first user message
- Text files wrapped in <file name="path">content</file> tags
- Images (.jpg, .jpeg, .png, .gif, .webp) attached as base64-encoded attachments
- Supports ~ expansion, relative and absolute paths
- Empty files are skipped silently
- Non-existent files cause immediate error with clear message
- Works in interactive, --print, and --mode text/json modes
- Not supported in --mode rpc (errors with clear message)

Examples:
  pi @prompt.md @image.png "Do this"
  pi --print @code.ts "Review this code"
  pi @requirements.md @design.png "Implement this"

Closes #54

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- **CLI File Arguments (`@file`)**: Include files in your initial message using the `@` prefix (e.g., `pi @prompt.md @image.png "Do this"`). All `@file` arguments are combined into the first message. Text files are wrapped in `<file name="path">content</file>` tags. Images (`.jpg`, `.jpeg`, `.png`, `.gif`, `.webp`) are attached as base64-encoded attachments. Supports `~` expansion, relative/absolute paths. Empty files are skipped. Works in interactive, `--print`, and `--mode text/json` modes. Not supported in `--mode rpc`. ([#54](https://github.com/badlogic/pi-mono/issues/54))
+
 ### Fixed
 
 - **Editor Cursor Navigation**: Fixed broken up/down arrow key navigation in the editor when lines wrap. Previously, pressing up/down would move between logical lines instead of visual (wrapped) lines, causing the cursor to jump unexpectedly. Now cursor navigation is based on rendered lines. Also fixed a bug where the cursor would appear on two lines simultaneously when positioned at a wrap boundary. Added word by word navigation via Option+Left/Right or Ctrl+Left/Right. ([#61](https://github.com/badlogic/pi-mono/pull/61))