feat: Add a button to create a new OpenCode session

[gmcheck]

If OpenCode has never been started, there's nowhere to create a new OpenCode session.

[kriss-spy]

there is an obsidian command called Opencode: New session
other commands also open an opencode tab if it's not running
are you requesting a ribbon button?

[gmcheck]

Yes. From a plugin usage perspective: since there is already a session sync button, if I don’t want to use an existing session, clicking the button to create a new session still feels quite natural and aligned with common usage habits. Also, the current session sync feature syncs all of OpenCode’s historical sessions; as OpenCode is used over time, the number of sessions will grow very large and become messy. Personally, I think session sync here should only sync sessions that are used within Obsidian.

PS: The “Windows PTY support not yet implemented.” bug still exists.