fix(sessions): filter out child sessions from /sessions list

[CODe5753]

Description of changes

The /sessions command returns every session from the API, including child sessions
spawned by tool calls (look_at, task, etc.). This clutters the list and makes it
hard to find actual user sessions.
Passes roots: true to session.list() so the server only returns root sessions.
This is the same approach the official OpenCode CLI and web app already use.
Relates to #1.

How it was tested

• Confirmed roots: true is implemented server-side in OpenCode:
  • DB query: session/index.ts#L773-L774 — adds WHERE parent_id IS NULL
  • API route: server/routes/session.ts#L50 — Zod schema with roots param
• All existing tests updated and passing: npm run lint, npm run build, npm test (405/405)

Checklist

• PR title follows Conventional Commits: <type>(<scope>)?: <description>
• This PR contains one logically complete change
• Branch is rebased on the latest main
• I ran npm run lint, npm run build, and npm test
• If this PR is OS-sensitive, behavior/limitations for Linux/macOS/Windows are described

[grinev]

@CODe5753 thanks for cinrtibution!