feat: download artifacts matching glob patterns after each batch

[jsk11235]

Summary

• Adds download_globs config field to [report] section — a list of glob patterns for files to download from sandboxes after each batch execution
• After each batch, runs find -path in the sandbox to resolve globs (RT1), then downloads all matched files in a single sandbox.download() call (RT2)
• Downloaded files stored at {output_dir}/{sandbox_id}/{batch_id}/ preserving relative directory structure
• Best-effort: download failures are logged as warnings, never fail a batch

Config example

[report]
download_globs = ["artifact-output/**", "*.xml", "coverage/*"]

Files changed

• src/config/schema.rs — download_globs: Vec<String> on ReportConfig with #[serde(default)]
• src/orchestrator/runner.rs — build_find_command(), try_download_artifacts(), new fields + builders
• src/orchestrator/spawn.rs — wire batch_idx, download_globs, output_dir into runner
• examples/tests/test_artifacts.py — pytest tests that produce JSON, log, CSV, XML artifacts
• offload-pytest-artifacts.toml — e2e config exercising the feature

Test plan

• cargo fmt --check passes
• cargo clippy --all-targets --all-features passes
• cargo nextest run passes (134 tests including 6 new)
• E2E: ./target/release/offload -c offload-pytest-artifacts.toml --verbose run downloads 4 artifact files per sandbox to test-results-artifacts/{sandbox_id}/{batch_id}/artifact-output/
• Test on Modal provider (remote sandbox)

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