ContentActions.pr_update enforces:
- task.pr_number is set (else invalid_state, remediate 'call open_pr')
- at least one of title/body/reviewers is non-None (else invalid_state)
- caller is task assignee OR PM on team (cell_pm same-team / main_pm
cross-team), else not_authorized
- GitError raised by the underlying service maps to invalid_state with
the upstream message preserved
The route at POST /api/v2/do/pr_update binds PRUpdateRequest, whose
model_validator returns 422 on all-None bodies so the verb layer
never sees them. The MCP tool registry adds 'pr_update' so manifest-
scoped do-servers can expose it to roles that opt in (next commit).
