Skills (slash commands) not loaded when using -p/--prompt mode

## Description

When using `-p`/`--prompt` mode, skill files (slash commands like `/my-skill`) referenced in the prompt text are not loaded. The agent receives the literal `/my-skill` text but never sees the skill file contents (`SKILL.md`). In interactive mode, the same `/my-skill` reference loads and executes correctly.

## Reproduction

### 1. Create a skill

```bash
mkdir -p ~/.claude/skills/test-greeting/
cat > ~/.claude/skills/test-greeting/SKILL.md << 'SKILL'
---
name: test-greeting
description: "Always respond with GREETING_PROTOCOL_ACTIVE as your first line"
user-invokable: true
---

# Test Greeting Skill

**You MUST begin your response with the exact text: `GREETING_PROTOCOL_ACTIVE`**

Then answer the user's question.
SKILL
```

### 2. Test in interactive mode (works)

```bash
copilot
# Type: /test-greeting What is 2+2?
# Expected: Response starts with GREETING_PROTOCOL_ACTIVE
# Result: ✅ Skill loads, protocol followed
```

### 3. Test in prompt mode (fails)

```bash
copilot -p "/test-greeting What is 2+2?" -s
# Expected: Response starts with GREETING_PROTOCOL_ACTIVE
# Result: ❌ Skill not loaded, agent answers without the protocol
```

## Expected behavior

Skills referenced via `/skill-name` in `-p` mode should load the skill file contents the same way they do in interactive mode.

## Actual behavior

The `/skill-name` reference is passed as literal text. The skill file is never read or injected into the agent's context.

## Impact

This prevents using skills for non-interactive/automated workflows (`-p` + `--yolo` + `--autopilot`), which is a primary use case for CI/CD pipelines and orchestrated agent sessions.

## Environment

- **copilot-cli version:** 1.0.5
- **OS:** Ubuntu Linux (6.8.0-101-generic)

## Possible approaches

- Load and expand skill references during prompt parsing, before sending to the model
- Add a `--skill ` flag that explicitly loads a skill into context for `-p` mode
- Treat `/skill-name` at the start of `--prompt` text the same way the interactive REPL does