bug: Amazon Q CLI lacks persistent custom instruction functionality

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

MacOS Sequoia

### Expected behaviour

**Expected Behavior:**
Users should be able to set persistent custom instructions (like system prompts) that automatically apply to every Q CLI session, similar to ChatGPT's custom instructions feature. This should work reliably without complex workarounds.

**Impact:**
This is a fundamental UX limitation that prevents users from customizing Q CLI behavior for their workflows. Other AI tools provide this basic functionality. Without this feature, Q CLI is significantly less useful for professional development work.

**Suggested Solution:**
Add a command like `q settings instructions set "custom instructions here"` or similar functionality to persist user instructions across sessions.

**Developers Notes**
So much potential for q but simple functionality like this shows there is no care for a good user experience and was very disapointing to see I had to create an issue for something so basic and simple. If this cant be added, I cant see ever paying for monthly subscription. Currently im just having to do `/load` and direct q to a file that contains how i want it to work, that is an **INSANE** way to do things *every single time* I start up a new instance of q chat. PLEASE improve this. **User experience is very very important**

### Actual behaviour

1. Custom agents fail with tool validation errors even with minimal configurations
2. No built-in way to set persistent custom instructions
3. /load command expects conversation format, not instruction format
4. /context command only handles file context rules, not system instructions
5. Piping instructions as first message treats each instruction as a separate question to answer
6. Users must manually copy/paste instructions every session or use unreliable workarounds

### Steps to reproduce

1. Try to create a custom agent with basic instructions - fails with tool validation errors
2. Try to use /load with instruction file - fails with format errors
3. Try to use /context for system instructions - only works for file context
4. Try piping instructions as first message - Q responds to each instruction individually instead of treating them as system context
5. Result: No reliable way to set persistent custom instructions

### Environment

```yaml
q cli needs **heavy work** done to improve… even running q diagnostic doesnt give you actual diagnostic data, only a menu, here were the results after running q diagnostic:
> q diagnostic

🛠️ Using tool: introspect (trusted)
⋮
● Introspecting to get you the right information ⋮
● Completed in 0.0s

⠋ Thinking...
> I don't see a /diagnostic command in the Q CLI documentation. The available commands are:

• /quit - Quit the application
• /clear - Clear conversation history
• /agent - Manage agents
• /context - Manage context files
• /editor - Open editor to compose prompts
• /reply - Open editor with quoted assistant message
• /compact - Summarize conversation
• /tools - View tools and permissions
• /issue - Create GitHub issue
• /logdump - Create zip file with logs for support
• /changelog - View Q CLI changelog
• /prompts - View and retrieve prompts
• /hooks - View context hooks
• /usage - Show context window usage
• /mcp - See loaded MCP servers
• /model - Select conversation model
• /experiment - Toggle experimental features
• /subscribe - Upgrade to Q Developer Pro
• /save - Save current conversation
• /load - Load previous conversation
• /todos - View and manage TODO lists
• /paste - Paste image from clipboard
• /help - Print help message

If you're looking for diagnostic information, you might want:
• /logdump - Creates a zip file with logs for support investigation
• /tools - Shows available tools and their permissions
• /usage - Shows current context window usage

Could you clarify what diagnostic information you're looking for?

Reference: https://github.com/aws/amazon-q-developer-cli/blob/main/docs/built-in-tools.md
```