#621 Add a way to list registered report definitions (report search only lists results)

Problem

swamp report search [query] lists stored report results (artifacts from past runs), not the report definitions currently registered/loaded from extensions/reports/. After authoring a new report (for example a workflow-scope report), there is no direct command to confirm it loaded and is selectable before running anything — report search returns an empty result set until the report has executed at least once and produced a stored artifact.

Steps to reproduce

1. Add a new report under extensions/reports/ (e.g. a workflow-scope report with a fresh name).
2. swamp report search <name> --json -> { "issues"/"results": [] } because the report has never run, even though it is correctly registered (it does run and produce output once the workflow executes).
3. There is no swamp report list / --definitions equivalent. swamp doctor extensions confirms extensions load but does not enumerate report definitions, and swamp model get <model> --json shows the resolved report set only for reports attached to a specific model.

Proposed solution

Add a way to list registered report definitions — name, scope, and source file/extension — distinct from stored results. For example a swamp report list command, or a reports section in swamp doctor extensions --verbose. This lets authors verify a report registered before its first run.

Alternatives considered

Run the workflow/method once, then swamp report search — but that conflates "is it registered" with "has it produced output", and gives no signal if the report failed to load.

Environment

swamp 20260608.234005.0-sha.ed5f78a4.