Feature Request: Add built-in auto-merge keybinding support

[k-jun]

Is your feature request related to a problem? Please describe.
When reviewing multiple PRs in gh-dash, enabling auto-merge requires leaving the dashboard to run gh pr merge --auto. This breaks the workflow and adds friction, especially when batch-processing PRs that are waiting on CI.

Describe the solution you'd like
A built-in keybinding (e.g., Shift+M) that enables auto-merge on the currently selected PR, with a confirmation prompt. Ideally it would support merge strategy selection (squash/merge/rebase) either via config default or an inline picker.

Additionally, the PR detail sidebar (Overview tab) should display whether auto-merge is enabled. Currently, the Checks Overview section in prview/checks.go renders three status categories — Review, Checks, and Merge — but none of them reflect auto-merge state. The proposed UI change:

• Add an auto-merge status indicator in the Merge Status section of the Checks Overview box. When auto-merge is enabled, show the selected merge strategy (e.g., Auto-merge enabled (squash)) with a distinct icon and color, alongside the existing merge state (CLEAN, BLOCKED, CONFLICTING, etc.).
• This requires fetching the autoMergeRequest field from the GitHub GraphQL API in data/prapi.go, which provides enabledAt, mergeMethod, and enabledBy.

Describe alternatives you've considered
Custom keybindings with command field work today. A toggle-style command can enable or disable auto-merge depending on the current state:

keybindings:
  prs:
    - key: M
      command: >
        AUTO=$(gh pr view {{.PrNumber}} -R {{.RepoName}} --json autoMergeRequest -q '.autoMergeRequest // empty');
        if [ -n "$AUTO" ]; then
          gh pr merge --disable-auto {{.PrNumber}} -R {{.RepoName}};
        else
          gh pr merge --auto --merge {{.PrNumber}} -R {{.RepoName}};
        fi

This is functional but has limitations:

• No visual feedback in the dashboard (the PR status doesn't update until refresh)
• No confirmation prompt, so accidental keypresses can toggle auto-merge
• Every user must configure it manually
• The toggle logic is non-trivial for a keybinding — a built-in command would handle this seamlessly

Additional context
Auto-merge is a core GitHub feature that pairs naturally with a PR dashboard workflow. Having it built-in would also allow gh-dash to show auto-merge status as a visual indicator on the PR list.