fix(workflows): guard directory-shaped workflow.yml and unreadable registry

- workflow add's plain local-path fallback (no --dev) checked wf_file.exists()
  before installing, so a directory literally named workflow.yml passed the
  guard and _validate_and_install_local() leaked an uncaught
  IsADirectoryError instead of the documented CLI error. Use is_file(),
  matching the --dev branch's existing guard.
- WorkflowRegistry._load() treated any OSError while reading an existing
  registry the same as corrupted JSON, resetting to an empty in-memory
  registry. A later save() would then silently persist that empty state via
  os.replace, discarding every previously installed workflow entry. Track a
  _load_error flag on OSError-during-read and have save() refuse to write
  when it is set, so a transient I/O failure can no longer overwrite intact
  data on disk.
- docs/reference/workflows.md: document `--from <url>` with its value
  placeholder, matching extensions.md and presets.md.

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docs/reference/workflows.md

@@ -86,10 +86,10 @@ Lists workflows installed in the current project.
 specify workflow add <source>
 ```
 
-| Option   | Description                                            |
-| -------- | ------------------------------------------------------ |
-| `--dev`  | Install from a local workflow YAML file or directory   |
-| `--from` | Install from a custom URL (`<source>` names the expected workflow ID) |
+| Option          | Description                                            |
+| --------------- | ------------------------------------------------------ |
+| `--dev`         | Install from a local workflow YAML file or directory   |
+| `--from <url>`  | Install from a custom URL (`<source>` names the expected workflow ID) |
 
 Installs a workflow from the catalog, a URL (HTTPS required), or a local file path.