feat: Wire up initiative deletion end-to-end

Add deleteInitiative tRPC procedure, wire Delete button in InitiativeCard
with confirm dialog (Shift+click bypass), remove unused onDelete prop chain.
Fix agents table FK constraints (initiative_id, account_id missing ON DELETE
SET NULL) via table recreation migration. Register conversations migration
in journal. Expand cascade delete tests to cover pages, projects, change
sets, agents (set null), and conversations (set null).

docs/server-api.md

@@ -88,6 +88,7 @@ Each procedure uses `require*Repository(ctx)` helpers that throw `TRPCError(INTE
 | listInitiatives | query | Filter by status |
 | getInitiative | query | With projects array |
 | updateInitiative | mutation | Name, status |
+| deleteInitiative | mutation | Cascade delete initiative and all children |
 | updateInitiativeConfig | mutation | mergeRequiresApproval, executionMode, branch |
 
 ### Phases