feat(cli): prompt for updates on startup

[Shengming Liang (LiangShengming)]

Pull Request

Summary

What:

Add a startup update prompt for interactive Dedalus CLI launches. The prompt uses a cached latest-version check, skips noninteractive/CI/help/completion/root update flows, and delegates accepted updates to the existing dedalus update implementation.

Why:

Users should not have to remember to run dedalus update --check manually. The goal is to keep lightweight latest-version state on disk, prompt only when there is a newer version, and route the accepted update through the installer-aware update path.

Test Plan

• git diff --check
• go test -run 'Test(StartupUpdate|RootCommandArg|IsNewerVersion|Update|Latest)' ./pkg/cmd
• go build ./...
• ./scripts/lint
• ./scripts/test

Repro / Showcase

When an interactive launch has a cached or freshly fetched newer version, the CLI prints:

A new Dedalus CLI is available.

Current: v0.4.0
Latest:  v0.5.0

Update now?
> Yes
  Not now

Press Enter to update, or type n then Enter to skip:

Pressing Enter runs dedalus update and exits after the update attempt. Typing n continues to the original command. Noninteractive launches, CI, help/version commands, completion generation, and dedalus update itself skip the startup prompt.

Tests Added

• Unit tests
• Integration tests
• E2E tests
• N/A (no new code paths)

Risk areas:

• The launch path now may perform a bounded latest-version fetch when the cache is stale or missing; the timeout is 2 seconds and failures are silent.
• The prompt writes to stderr so stdout remains usable for command output.
• The prompt intentionally has only Yes and Not now

[Shengming Liang (LiangShengming)]

End to end evidence: run 28068838181 passed.