Add opt-in periodic CLI update notices

[chaptersix]

Closes #235.

What changed

• add a profile-independent cli.update_check.enabled config property for opting in
• query temporal.download for the latest stable CLI release on a jittered 60–84 hour cadence
• cache only the latest version and scheduling metadata in the existing Temporal TOML config
• emit newer-version notices to stderr with 1, 3, 7, then 14-day reminder backoff
• include the update check in all CLI invocations, including Cobra's --version path
• preserve CLI-owned top-level config when connection profiles are rewritten
• suppress update-check network and config failures so normal CLI behavior is unaffected

User impact

Users can opt in with:

temporal config set --prop cli.update_check.enabled --value true

Once enabled, the CLI periodically checks for a newer release and prints a throttled notice to stderr without changing structured stdout.

Validation

• go test ./internal/temporalcli -count=1
• go test ./... -count=1
• git diff --check