feat(analytics): add PostHog analytics for CLI command tracking

[theFong]

Summary

• Adds PostHog SDK integration for comprehensive, privacy-conscious CLI analytics
• Captures every command invocation generically via PersistentPostRunE on the root command
• Users are prompted to opt in during brev login, with preference persisted in ~/.brev/personal_settings.json
• Anonymous users get a stable UUID; on login, posthog.Alias links anonymous and real user IDs

Tracked Properties

Property	Source
command, args, flags	Cobra command context
succeeded, duration_ms	Execution result + timer
cli_version, go_version	Build/runtime info
os, arch, num_cpus	runtime package
cwd	os.Getwd()
is_tty, is_stdin_piped	File descriptor stat
shell, terminal	$SHELL, $TERM
is_ci	Checks CI, GITHUB_ACTIONS, GITLAB_CI, etc.
is_ssh	$SSH_TTY, $SSH_CONNECTION
locale, timezone	$LANG / time.Now().Zone()
gpu_info	nvidia-smi or system_profiler on macOS
parent_process, parent_cmdline	Parent PID inspection

Test plan

• go build compiles successfully
• Run brev login — should prompt for analytics preference after auth
• Check ~/.brev/personal_settings.json has analytics_enabled and analytics_id
• Run brev ls — if opted in, cli_command event should appear in PostHog
• Run brev login again — should NOT re-prompt (preference already set)
• Run invalid command — error event should fire with succeeded: false
• Existing tests pass