Lint with expanded workflows to validate URL/import subworkflows

## Summary

The linter should support an expanded mode that uses `expanded_format2()` / `expanded_native()` to resolve all `@import` and URL references before linting. This would validate the structure of externally-referenced subworkflows, not just inline ones.

## Context

We're adding recursive linting for inline subworkflows (steps with `run: {class: GalaxyWorkflow, ...}`). This catches structural issues like dangling `outputSource` references in nested workflows.

However, subworkflows can also be referenced via:
- `@import` — relative file path
- URL strings — `http://`, `https://`, TRS URLs
- `$graph` references — `#subworkflow_id`

These are currently opaque to the linter. The normalized models already have `ImportReference` and `str` types for these, and `expanded_format2()` resolves them into inline `ExpandedFormat2` subworkflows.

## Proposed Approach

Add a `lint_format2_expanded()` (or a `--expand` flag to the CLI) that:
1. Calls `expanded_format2(workflow, options)` with a configured `url_resolver` and `workflow_directory`
2. Walks the fully-expanded tree and lints each subworkflow recursively
3. Reports errors with context about which import/URL they came from

## Considerations

- Requires filesystem access for `@import` and network access for URL resolution
- Should be opt-in since it has side effects (network, filesystem)
- `ConversionOptions` already supports `url_resolver` and `workflow_directory`
- `expanded_format2` already handles cycle detection and max depth
- Could integrate with IWC lint tests which already have local workflow files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start by reading expanded_format2(), ConversionOptions, and the existing recursive linting for inline subworkflows. Trace how IWC lint tests provide local workflow files, then define an opt-in expanded lint path that resolves imports and URLs, lints the expanded tree with source context, and preserves cycle and depth handling.

Written by the indexing model from the issue text.