Enhancement: pi-rust adapter should support --provider and --append-system-prompt flags #82

Context

Pi-rust supports --provider, --append-system-prompt, and --no-prompt-templates flags, but the agentctl pi-rust adapter doesn't pass them through.

Why it matters

• --provider: needed when model ID alone is ambiguous across providers
• --append-system-prompt <file>: critical for injecting project context (AGENTS.md) for fair model comparisons
• --no-prompt-templates: useful for controlled trials

Current workaround

Prepend AGENTS.md content into the prompt text itself.

Suggested

Add --provider passthrough from opts.adapterOpts or model string prefix parsing. Add --append-system-prompt support via a new launch flag or adapter option.