fix(core): use current workspace with /new; fix warping into local project

[jlongster]

Issue for this PR

N/A

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes a few workspace follow-up issues in the TUI:

• Uses the selected /warp workspace when creating a new session from the home prompt instead of falling back to the local project.
• Defaults /new sessions to the current workspace when the TUI is already scoped to one.
• Filters workspace pickers to connected workspaces before choosing the three most recent entries.
• Preserves null when warping a session back to the local project, and updates the generated SDK type so the API contract allows id: null.

These work because session creation now derives its workspace from the explicit warp selection first, then the current TUI workspace, while local-project warp sends the server's expected null value instead of omitting id.

How did you verify your code works?

Ran bun typecheck from packages/opencode.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR