feat: add update command and passive version-check notification

[debidong]

Summary

• Add flashduty update command that checks GitHub Releases for the latest version and shells out to the existing install script (install.sh / install.ps1) to perform the upgrade
• Add flashduty update --check flag for check-only mode (no install)
• Add background version-check notification: every command silently checks GitHub Releases (24h interval), then prints a non-blocking stderr reminder if a newer version exists
• Change goreleaser draft: true to draft: false so the /releases/latest API endpoint returns published releases

Gating logic (passive check is skipped when)

• Version is dev or (devel) (local build)
• FLASHDUTY_NO_UPDATE_CHECK=1 is set
• CI environment detected (CI, GITHUB_ACTIONS, JENKINS_URL, GITLAB_CI)
• Last check was less than 24 hours ago (persisted in ~/.flashduty/state.yaml)
• stderr is not a TTY
• Command is update or version

New files

• internal/update/check.go — version check logic, state file, GitHub API, semver comparison
• internal/update/check_test.go — 14 test cases
• internal/cli/update.go — flashduty update Cobra command

Modified files

• internal/cli/root.go — PersistentPreRun/PersistentPostRun hooks, register update command
• .goreleaser.yml — draft: false

Test plan

• go test -race ./internal/update/... passes
• go test -race ./... passes
• flashduty update --check prints current vs latest version
• flashduty update downloads and installs latest version via install script
• Running any command (e.g. flashduty version) does NOT trigger background check (excluded command)
• Running a regular command shows update notice on stderr if newer version exists
• FLASHDUTY_NO_UPDATE_CHECK=1 suppresses the background check
• Running twice within 24h skips the API call on second run