feat(opencode): add option to mark sessions as favorite.

[sergio-asenjo]

Issue for this PR

Closes #10854

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds the ability to add a session to favorites, and a toggle to filter through/show them.

How did you verify your code works?

Tested locally, against opencode's current version.

Screenshots / recordings

opencode-fav-thingy.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, the most relevant finding is:

PR #15025 - "fix(tui): show models in provider section even when in recents/favorites"
#15025

This PR mentions "favorites" functionality and appears to be related to session management/filtering. It may be addressing the same feature area or could be a dependent PR that works with the favorites system being introduced in PR #15138.

No other clear duplicates found that are directly addressing the "mark sessions as favorite" feature itself.

[codenoid]

oh my god, I was about to implement a same thing, why don't we have this merged, cc @rekram1-node

[rekram1-node]

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