Add get prompt with arguments functionality

[kashifpk]

Summary

Add the ability to get/execute prompts with arguments, not just list them.

Background

Currently Anvil can list available prompts but cannot retrieve prompt content with arguments. The MCP protocol supports prompts/get to fetch rendered prompt content.

Proposed Implementation

CLI

# Get a prompt (no arguments)
anvil get-prompt "code-review"

# Get a prompt with arguments
anvil get-prompt "code-review" --arg language=python --arg style=detailed

# With JSON arguments
anvil get-prompt "summarize" --json-args '{"text": "...", "length": "short"}'

# JSON output
anvil get-prompt "code-review" --json

Web UI

• Add "Get" or "Execute" button for each prompt in the list
• When clicked, show a form with argument fields (similar to tools)
• Display the rendered prompt content (messages array)
• Include description and any metadata

Technical Implementation

async def get_prompt(self, name: str, arguments: dict[str, str] | None = None) -> dict[str, Any]:
    """Get prompt content with optional arguments."""
    async with Client(self.server_url) as client:
        result = await client.get_prompt(name, arguments or {})
        return {
            "description": result.description,
            "messages": [
                {
                    "role": msg.role,
                    "content": _content_to_list(msg.content),
                }
                for msg in result.messages
            ],
        }

Acceptance Criteria

• CLI command anvil get-prompt <name> implemented
• Support for --arg key=value arguments
• Support for --json-args for complex arguments
• Web UI shows argument form based on prompt schema
• Rendered messages displayed clearly
• Error handling for missing required arguments

References

• MCP prompts/get spec: https://spec.modelcontextprotocol.io/specification/server/prompts/#getting-a-prompt