feat(agentic-epic/graph): same-scope epic de-dup + child in-flight guard [po-formulas-software-dev-2eb]

[ryanholtschneider2]

Two hardening items from the soloco-kol8q runaway incident:

1. Same-scope epic de-dup (agentic_epic.py):

   • _scope_fingerprint(): sha1 of first 400 chars of normalized goal text
   • _acquire_scope_guard(): writes a rig-level sentinel file; returns
     conflicting epic_id if another in-progress epic holds the same scope
   • _release_scope_guard(): removes the sentinel on flow exit (finally block)
   • agentic_epic flow: checks the guard before decomposition; returns
     skipped-same-scope when a duplicate fan-out would happen
   • force_replan=True and dry_run bypass the guard (deliberate re-runs)

2. graph_run child-level in-flight guard (graph.py):

   • _live_flow_run_count(): queries Prefect for Running flow runs tagged
     with issue_id:; returns 0 on error (graceful degradation)
   • _child_is_in_flight(): two-stage check — bead status then Prefect query
   • _dispatch_nodes(): skips any child that is in_progress + has a live
     flow run; adds it to the skipped dict with an 'already in-flight' reason