DEV: Rename plugin to "Process Manager"

[merefield]

Summary

This PR completes the rebrand from Workflow to Process Manager across the plugin.

This was necessary to avoid a major imminent naming clash with a new core plugin.

It renames the plugin identity, routes, settings, locale namespace, frontend/admin assets, Ruby namespace, models, serializers, specs, fixtures, and runtime payloads to use Process Manager / Process terminology. It also adds a post-migration to rename the underlying workflow tables and columns to process_manager_* equivalents, with AR associations
and SQL paths updated accordingly.

Key Changes

• Renamed plugin-facing copy, admin navigation, discovery route, settings, assets, CSS selectors, DOM attributes, and
  frontend internals from Workflow to Process Manager / Process.
• Renamed Ruby namespace and helper/model/serializer/controller usage to ProcessManager.
• Added schema migration from legacy workflow_* tables/columns to process_manager_* tables/columns.
• Updated Data Explorer default query names and SQL to use Process Manager naming while leaving legacy query names
  untouched.
• Updated README and admin help copy, including direct OpenAI AI-step wording and current visual/Kanban/chart
  capabilities.

Validation

• Full plugin spec suite passed.
• Lint passed for touched files.
• Test DB migration completed successfully.