[FEATURE]: Add --worktree / -w startup flag to V2

Feature hasn't been suggested before.

• I verified that no active issue requests this exact feature. This renews #21680 and #35471, which were automatically closed after 60 days of inactivity.

Describe the enhancement you want to request

Add a --worktree / -w startup flag to OpenCode V2. The flag must create or select an isolated git worktree before it starts the TUI session.

opencode2 -w feature-a
opencode2 --worktree feature-b
opencode2 --worktree

Expected behavior:

• Create or select a named worktree when the user supplies a name.
• Generate a worktree name when the user omits the value.
• Start the session with that worktree as its working directory.
• Keep session history, file state, tools, and MCP state isolated from sessions in other worktrees.
• Reuse the existing V2 worktree and workspace infrastructure.

This supports parallel tasks without manual worktree setup or extra terminal tabs. It also gives Claude Code users an equivalent to its -w workflow.

This request differs from #36048. That issue requests worktree subcommands and in-session switching with stash-based state management. This issue requests a startup flag for a one-command isolated session.

Related issues:

• #21680 — original request, closed after inactivity
• #35471 — detailed CLI flag request, closed after inactivity
• #36048 — in-session worktree commands and switching
• #33332 — configurable worktree creation