[FEATURE]:Plugin API for custom status bar / sidebar indicators #36625

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

Problem

The TUI status bar currently shows only fixed built-in indicators:
session name, context %, MCP status, LSP status.

There's no way for users or plugins to add custom indicators.
This makes it impossible to display project-specific or plugin-provided
information at a glance.

Use cases I have in mind

1. Show how many skills are loaded (global + project)
2. Display active MCP connections with online/offline health
3. Show current git branch and uncommitted changes count
4. Custom build/test status from a CI plugin
5. Plugin-specific state (e.g., "Code review: 3 pending")

Proposed API

Expose status bar registration via plugin hooks:

// Register a custom indicator
tui.statusbar.register({
  id: "skills-count",
  label: "Skills",
  render: () => `${loadedSkills.length}`,
  tooltip: "Global: 3, Project: 2",
  order: 10, // position in bar
})
Or simpler declarative form:
return ### {
  "tui.statusbar.item": {
    id: "git-branch",
    label: "Branch",
    value: async () => await $`git branch --show-current`.text(),
  }
}
Alternatives considered
- Currently zero workaround: toast notifications aren't suitable for persistent status
- The only TUI hooks are tui.prompt.append, tui.command.execute, tui.toast.show — none relate to layout
Priority
Medium. Not blocking, but would significantly improve the plugin ecosystem
and let users tailor the TUI to their workflow.