📏 Add prompt content length limits

[jordanpartridge]

Problem

Assembled prompts might exceed GitHub comment limits (65,536 characters), causing API failures.

Current Risk

File: app/Services/PromptAssembler.php

• No limit checking on assembled prompt content
• GitHub API will reject oversized comments
• Users lose actionable feedback

Proposed Implementation

if (strlen($assembled) > 60000) {
    $assembled = substr($assembled, 0, 60000) . 
                "\n\n... (truncated, see full output in Actions tab)";
}

Benefits

• Prevents GitHub API rejections
• Ensures users always get some actionable feedback
• Graceful degradation for large outputs