Point users at shell completion setup on first start

[joe4dev]

Motivation

Only Homebrew installs get shell completion set up automatically (homebrew_casks.completions). npm and GitHub-release users have to find the docs themselves, so a shipped time-saving feature goes mostly unused.

Solution

Emit one line after the first successful interactive start, pointing at lstk completion [bash|zsh|fish|powershell] and the docs section:

> Tip: Enable tab completion for your shell: lstk completion [bash|zsh|fish|powershell] See https://docs.localstack.cloud/aws/developer-tools/running-localstack/lstk/#shell-completions

Two deliberate deviations from the ticket: the trigger is the first run, not install (no install path offers a usable hook — npm's package.json is generated, a postinstall that edits a shell rc is hostile and is skipped under --ignore-scripts, and binary installs have no hook), and it is a pointer, not a prompt — lstk never writes to the user's shell config. An automated lstk completion --write was designed and dropped in favour of this because the big complexity (cross-platform, cross-shell, cross-install-method) is not worth maintaining for a small gain.

Needs no new persisted state: firstRun means "config.toml was absent", and that same path creates the config, so the tip cannot repeat. Interactive-only, so CI and --json output are unchanged. Prefix and severity match tipsForType, whose tip renders directly above it.

The wording follows the Verb ...: <command> shape the neighbouring tips already use.

The main_test.go/awsconfig_test.go part of the diff is test plumbing: these tests isolate HOME under t.TempDir() and start a real emulator, whose container writes root-owned files into the bind-mounted volume that Go's TempDir cleanup cannot delete on Linux. awsconfig_test.go already solved this; scheduleVolumeCleanup extracts it so there is one copy rather than two.

Manual testing

make build
export LOCALSTACK_AUTH_TOKEN=<token>
H=$(mktemp -d)

HOME=$H bin/lstk start   # accept AWS at the emulator picker
#   -> last line is the "> Tip: Enable tab completion ..." line above

HOME=$H bin/lstk start   # tip must NOT reappear (config now exists)
HOME=$H bin/lstk stop

HOME=$(mktemp -d) bin/lstk start --json   # envelope must contain no tip

Screenshot from manual testing:

• Tip appears on first launch
• Tip does not appear on subsequent commands

Docs

No documentation change needed — the tip points at the existing "Shell completions" section and adds no command, flag, or env var. Worth knowing that the CLI now links to that section directly, so the #shell-completions anchor should stay stable.

Review

Human review advised — new user-facing output on the shared start path.

Todo

• Manual testing (see above)

Closes DEVX-821