Introduce flows/workplan.yaml and /flows/workplan/* routes alongside the
legacy workstream flow. Dashboard, README, and MCP flow docs now prefer
open_workplans with open_workstreams fallback until dual-key retirement.
Add the ranked legacy-interface backlog (T01), rename dashboard navigation and
user-facing copy to workplan while preserving wire-compat API keys (T02), and
activate the retirement workplan with T01/T02 marked done.
