[FEATURE]: add projects listing command

[allen-n]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Summary

Basically, I wanted to know where all the opencode projects on my machine are and which ones have activity, especially when I don’t remember where I have or haven’t used it

Proposal:

• add opencode projects cli command to list known projects with session counts, last activity, and worktree status
• support sorting (path|count|activity), limiting results, and filtering to active projects
• format output with truncated paths and localized timestamps for readability