Fix invalid YAML frontmatter in prompt files

[SameerYoussef]

Files:

• .github/prompts/init-populate-octofit_db.prompt.md
• .github/prompts/create-express-logic-tier.prompt.md

Issue: The prompt files use incorrect YAML frontmatter keys that don't match the official GitHub Copilot prompt file syntax.

Current (incorrect) syntax:

---
mode: 'agent'
model: GPT-5.5
description: 'Configure MongoDB and seed octofit_db for the Octofit multi-tier application'
---

Correct syntax (per GitHub docs):

---
agent: 'agent'
description: 'Configure MongoDB and seed octofit_db for the Octofit multi-tier application'
---

Changes needed:

1. Line 2: Replace mode: 'agent' with agent: 'agent'
2. Line 3: Remove the entire model: GPT-5.5 line (not a valid key)

This change should be applied to both prompt files in .github/prompts/