Remove the Squadron server, contract and client-runtime surface #464

Part of #412. Milestone: Squadron fold. Depends on: ledger re-key, J5 reads by project, agent tools, archive same-project rule, Playbook Author picks a project.

Outcome

Nothing in the server, contracts or client-runtime refers to Squadrons. The FORK cases that existed only for Squadrons are gone, and the touched upstream files are back to the pin.

Scope

• Server. Delete SquadronHttp and ImportedThreadsHttp (assign-imported), SquadronManagementService, SquadronProjectReferences, SquadronThreadCreationService, SquadronLaunchPolicy, HomeRegistrar and ThreadHomesService (or reduce them to projectId), the CLI a2a Squadron commands, and J5SquadronCreationLayer.
• Upstream seams back to the pin:
  • ThreadLaunchService.ts registration hook (case 10)
  • ws.ts squadronId forwarding
  • ServerEnvironment.ts capability
  • ScheduledTaskService.ts shared-project guard from fix(scheduled-tasks): a scheduled task can start a fresh thread again #431 (case 12)
• Contracts. Remove squadronId from OrchestrationV2ThreadLaunchInput. Remove the client half of sourcePlanRef (case 11), so plan→implementation returns to upstream's create-then-start. Remove SquadronLaunchRequiresBootstrapError, J5_API_PATHS Squadron entries and j5SquadronActionPath.
• Client-runtime. Drop the ./j5/squadrons export.
• FORK.md. Remove cases 1/5/9–13/15b–20/23/34/38/39/43 and the rest of the Squadron-only cases as their code goes. Correct the case count.

Done when

• git grep -i squadron -- apps packages returns only migration 030 and earlier migrations.
• The diff of each touched upstream file against the pin is empty, or matches what its remaining FORK case records.