Add legacy_compat helper and Deprecation headers when callers filter
/tasks, /decisions, /token-events, or /execution/launch-requests with
workstream_id. Preferred workplan_id filters are unchanged. Route removal
remains gated on legacy-meter zero-usage windows.
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.
