Make upgrade command version check non-blocking with GitHub API

[Copilot]

The upgrade command now checks for newer releases via GitHub API and warns users without blocking execution. Previously used gh extension upgrade --dry-run which blocked upgrades when updates were available.

Changes

• Modified ensureLatestExtensionVersion() to query repos/github/gh-aw/releases/latest via go-gh REST client
• Changed from error to warning - returns nil even when newer version exists, allowing upgrade to proceed
• Silent failure on all errors - network issues, auth failures, and blocked URLs no longer interrupt workflow
• Added getLatestReleaseVersion() helper using existing Release type from update_check.go

Behavior

Before:

ERROR: gh-aw extension must be upgraded before running this command

(exits with error, blocks upgrade)

After:

⚠️  A newer version of gh-aw is available: v0.20.0 (current: v0.19.0)
ℹ️  Consider upgrading with: gh extension upgrade github/gh-aw

Updating agent and prompt files...

(shows warning, continues with upgrade)

Dev builds and pre-release versions skip the check entirely.

Original prompt

The upgrade command check if the current release matches the latest release of the extension and issue a warning if running gh extensions upgrade github/gh-aw is needed.
Fail silently if the release URL is not available or blocked.

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