fix(cli): make multi-project the default for agentv serve

[christso]

Problem

agentv serve --multi requires a flag to show the multi-project dashboard. This is unintuitive — if a user has registered multiple projects, they should see all of them by default. The --multi flag adds friction and is easy to forget.

Current behavior

• agentv serve → shows single-project view (current directory or most recent run)
• agentv serve --multi → shows multi-project dashboard with all registered projects

Proposed behavior

• agentv serve → auto-detects:
  • If multiple projects are registered in ~/.agentv/projects.yaml → multi-project dashboard
  • If only one project (or none, using cwd) → single-project view
• agentv serve --single → force single-project view (escape hatch)
• Deprecate --multi flag (keep it as no-op for backward compat, warn on use)

Also

• studio is an alias for serve — both should behave the same
• Consider whether studio should be the primary name and serve the alias, or if one should be removed

Acceptance criteria

• Multi-project dashboard is default when multiple projects exist
• Single-project still works for single-project users
• --multi deprecated with warning
• --single flag added as escape hatch