Nudge the agents for latest version upgrade (#98640)
Stacked on #98633.
This PR adds `experimental.agenticAutoUpgrade = 'latest'` config which
enables nudging the agents to notify the user when there's new
major/minor Next.js version available to upgrade. The nudge will include
guiding to upgrade via `next upgrade --ai` (run "latest" by detecting
config).
The method of nudging leverages the agents behavior where they tend to
listen to messages from fatal errors that blocks the process compared to
general error/warning logs. Whenever the agents run `next dev` or `next
build`, Next.js will detect the condition and nudge the agent using this
method.
Afterwards it's up to the user whether to proceed the upgrade or not,
it's 100% up to the user how to run it e.g. subagent, background agent,
etc. and the process should not enforce any that affects user's
workflow.
Enabling `experimental.agenticAutoUpgrade = 'latest'` also enables
security check.
