[FEATURE]:unarchive sessions

Feature hasn't been suggested before.

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Describe the enhancement you want to request

When a session is archived, there is no way to unarchive it from the UI or CLI:

No keybind exists in the TUI for archiving/unarchiving sessions (checked the keybinds list — there is no session_archive / session_unarchive entry).
opencode session list does not show archived sessions, and there is no opencode session unarchive command.
The archive action only sets the time_archived field on the session row in opencode.db. The only workaround is manually clearing that field via direct DB access, which is not acceptable for regular users.

Proposed solution

Add an unarchive option, e.g.:

a TUI keybind (session_archive / session_unarchive) that toggles archive state, or an option in the session list UI to show archived sessions and restore them;
a CLI command like opencode session unarchive .
Steps to reproduce

Archive a session from the session list.
It disappears from opencode session list.
There is no supported way to restore it.
Environment

opencode version: 1.18.15
OS: Windows