[codex] Default Studio to projects dashboard

[christso]

What changed

• default Studio to the projects dashboard unless --single is explicitly used
• auto-register and prefer the cwd-backed project when Studio launches inside a directory that contains .agentv
• redirect to that project only on the first visit to /, so users can still navigate back to the Projects home screen afterward
• reuse the existing project dashboard APIs instead of relying on separate single-project default behavior

Why

The previous default routing was inconsistent:

• neutral launches did not reliably land on the projects list
• project-directory launches effectively behaved like a hidden single-project mode
• the root route kept redirecting to the current project, which made the projects home screen harder to reach

Impact

• multi-project Studio now has a consistent default landing page
• launching from inside a project still gives a convenient first-open experience
• users can return to the Projects home after that initial redirect

Validation

• bun test apps/studio/src/lib/navigation.test.ts
• bun --filter @agentv/studio build