feat(cli): add experimental agents TUI (#24150)

> This PR was authored by Mux on behalf of Mike.

Adds `coder exp agents`, an interactive terminal UI for managing Coder
AI agent chats. Built with bubbletea/lipgloss/glamour, the TUI provides
parity with the web dashboard for chat management, model selection, and
real-time tool execution visibility.

## What it does

- **Chat list view**: tree-based navigation with nested subagent
expansion, search filtering, windowed scrolling, and pagination.
- **Active chat view**: viewport-based transcript with markdown
rendering, WebSocket streaming, and a text input composer for sending
messages.
- **Model picker overlay**: cached model catalog with fuzzy selection.
- **Diff drawer overlay**: git changes inspection with unified diff
rendering.
- **Tool call rendering**: humanized argument summaries, consecutive
duplicate collapsing, and status indicators.

## Key implementation details

- Session lifecycle uses a monotonic `chatGeneration` counter so async
responses from stale sessions are dropped on chat switch.
- Draft mode guards prevent duplicate chat creation on double-Enter.
- Error and loading states render inline without collapsing the TUI
chrome.
- Glamour renderer access is mutex-protected (not thread-safe).
- Intentional WebSocket close is distinguished from dropped connections
to prevent spurious reconnects.

## Testing

~220 unit tests covering rendering, state transitions, keyboard
dispatch, and edge cases. 4-scenario PTY-based E2E suite covers boot,
navigation, search, and direct chat open.

14 new files, ~7,400 lines added.

cli/root.go

@@ -164,6 +164,7 @@ func (r *RootCmd) AGPLExperimental() []*serpent.Command {
 		r.promptExample(),
 		r.rptyCommand(),
 		r.syncCommand(),
+		r.agentsCommand(),
 	}
 }