docs(site): document assignee fallback to last git author in create-work-item (#847)

When 'assignee' is omitted from the create-work-item configuration, the
executor falls back to the email of the last person who committed changes
to the agent source markdown file, discovered via 'git log' at Stage 3.
This behaviour was in docs/safe-outputs.md but missing from the site
reference page.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

site/src/content/docs/reference/safe-outputs.mdx

@@ -76,7 +76,7 @@ Creates an Azure DevOps work item.
 - `work-item-type` - Work item type (default: "Task")
 - `area-path` - Area path for the work item
 - `iteration-path` - Iteration path for the work item
-- `assignee` - User to assign (email or display name)
+- `assignee` - User to assign (email or display name). When omitted, falls back to the email of the last person who committed changes to the agent source markdown file (discovered via `git log` at Stage 3).
 - `tags` - Static list of tags always applied to the work item (regardless of agent input)
 - `allowed-tags` - Allowlist of tags the agent is permitted to use via the `tags` parameter. If empty, any agent-provided tags are accepted. Supports `*` wildcards anywhere in the pattern (e.g., `"agent-*"` matches `"agent-created"`; `"copilot:repo=org/project/*@main"` matches any repo name).
 - `custom-fields` - Map of custom field reference names to values (e.g., `Custom.MyField: "value"`)