feat: show a cached release update notice

[jwmoss]

Summary

• Show newer stable releases and archive replacement instructions on interactive launches.
• Cache release checks for 24 hours with a 500-millisecond network limit. Do not install updates.
• Skip JSON, plain, quiet, redirected output, CI, agent sessions, telemetry, and completion commands.
• Support DATATF_NO_UPDATE_NOTIFIER and DO_NOT_TRACK. Send no credentials or workspace data to GitHub.
• Add upgrade documentation and include it in release archives.

Validation

• make check
• CLI help reviewed
• Race tests for the CLI and release checker
• Windows AMD64 and Linux ARM64 cross-builds
• goreleaser check
• Interactive launch against GitHub: an older version label displays the current release
• Tests for cache expiry, version order, opt-out, invalid responses, timeouts, and unchanged exports

The repository has no prek configuration. The required Go checks pass.

Notes

This is an update notice, not a self-updater. Release checks are separate from optional telemetry.