Problem Description
When using the GitHub Copilot plugin in the Eclipse Git Staging view or commit window to auto-generate commit messages, the extension currently overwrites any existing text present in the commit message box. If a user has already written a draft, added a ticket number, or included custom notes, they lose their text entirely.

Additionally, custom commit instruction prompts currently lack support for runtime variables (such as the current date, time, or active branch name), limiting the flexibility of automated commit formatting.

Proposed Behavior
Non-Destructive / Append Behavior:

Before replacing the contents of the commit message box, the plugin should check if the field already contains text.

If text is present, instead of wiping it, it should append the auto-generated commit message at the end of the existing text (separated by a newline), preserving the user's manual input.

Dynamic Variable Support in Instructions:

Allow developers to use standard environment or IDE-style variables (e.g., ${current_date}, ${git_branch}) within their custom commit instruction templates.

The plugin should evaluate and inject these variables before sending the request or finalizing the commit text.

Use Case & Motivation
Prevents Accidental Data Loss: Users frequently type manual prefixes or contextual details into the commit box before hitting the AI generation button, only to have their work erased.

Streamlined Workflows: Supporting variables like dates or branch names allows teams to standardize conventional commits with precise temporal or structural context automatically.