[FEATURE] Agents View: show branch name, merge status, and mergeable state per session row

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Background sessions linked to a pull request are well represented in the agent view dashboard — the PR status is clearly shown. But if a workspace doesn't use a pull request workflow and instead develops locally through worktree branches that get merged into main/master, there's no way to track each session's branch state from the agent view session list. I have to step outside agent view and check the git branch state separately to figure out where each session stands.

Proposed Solution

For sessions without a PR, surface the equivalent git state directly in the row — at minimum the branch name and a compact merge/ahead-behind indicator against the base branch (something like worktree-xxxxx · 3 ahead or merged into main). Enough that I can decide from the dashboard whether to attach, merge locally, or safely delete, without leaving agent view.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_