RFC: Sidebar git branch display, API live indicator, and retry backoff

[camalolo]

Hi Charm team,

I've been working on a few small UI and agent improvements and wanted to share them for feedback before opening a PR,
per the contributing guidelines.

1. Git branch display in sidebar

Adds a "Git branch" section at the bottom of the sidebar that polls git rev-parse --abbrev-ref HEAD every 3 seconds
and renders the current branch name. This gives quick visibility into which branch you're on without leaving the TUI.
Reserves 2 lines of vertical space in the dynamic height calculation.

Files: internal/ui/model/git.go (new), internal/ui/model/ui.go , internal/ui/model/sidebar.go

2. Blinking API live indicator on model icon

Toggles the model icon to green every 500ms while the agent is busy making an API call, giving visual feedback that a
request is in progress. Starts and stops automatically based on agent busy state transitions. The ModelInfo common
element was updated to accept an optional iconOverride parameter.

Files: internal/ui/model/ui.go , internal/ui/model/sidebar.go , internal/ui/common/elements.go

3. Exponential backoff for API retries

Configures retry with MaxRetries=2 and OnRetry logging on the three provider call sites, so transient API failures
are automatically retried with increasing delays via fantasy's built-in backoff support. Adds an intPtr helper for the
*int configuration fields.

Files: internal/agent/agent.go , internal/agent/coordinator_test.go

All features include unit tests. The branch is pushed to camalolo/crush:improvements
https://github.com/camalolo/crush/tree/improvements.

Open to any feedback on the approach, naming, , or whether these are welcome additions.