feat: rename workstreams → SubSquads (community decision) #271

The community voted to rename "workstreams" (formerly "streams") to SubSquads.

This requires renaming across:

• CLI command: squad workstreams → squad subsquads (keep workstreams as deprecated alias)
• SDK types: WorkstreamDefinition → SubSquadDefinition (keep old types as deprecated aliases)
• SDK functions: loadWorkstreamsConfig() → loadSubSquadsConfig()
• Config file: .squad/streams.json → .squad/subsquads.json
• Activation file: .squad-stream → .squad-subsquad
• Env var: SQUAD_TEAM stays (generic enough)
• Governance: squad.agent.md SubSquad Awareness section
• Docs: all references
• Blog: update Self-testing framework: observe UX patterns and identify self-improvement opportunities #23 title
• Tests: update test names and assertions

Backward compatibility: keep deprecated aliases for at least one release cycle.