Add progress indicator for long-running model methods

[bixu]

Problem

When a model method iterates over many resources (e.g., 60 VMs), the CLI shows no output until the entire method completes. With sequential execution at ~6s per item, this means waiting several minutes with no feedback. Even with batched concurrency (PR #29), methods that touch dozens of resources still take 30-60s with zero visibility.

Proposed Solution

Add a progress indicator that surfaces method execution progress in real-time. Two complementary approaches:

1. Stream context.logger output during execution — model methods already log per-item progress (e.g., "checking VM 15/60"), but these logs are only shown after the method completes in --json mode. Streaming them to stderr in real-time would give immediate feedback.

2. Built-in progress API — provide a context.progress(current, total, message?) method that models can call to report progress. The CLI would render this as a progress bar or counter (e.g., [15/60] Checking VMs...).

Alternatives Considered

• Models could write partial results incrementally, but writeResource is per-item and the CLI still waits for the method to return.
• Users can watch swamp data list in another terminal, but this is awkward.

Implementation Notes

• Streaming logs to stderr would be a quick win and backward-compatible
• The progress API would need a new method on the execution context
• Both should work in --json mode (stderr for progress, stdout for final JSON)

[github-actions]

Hi @bixu, thanks for opening this issue!

We appreciate you taking the time to report it. A maintainer will review it as soon as possible.

In the meantime, please make sure you've included all relevant details (steps to reproduce, expected vs actual behaviour, swamp version, etc.) to help us triage it quickly.