feat(init): scaffold circleci init onboarding command

[Klaney]

Summary

Lands the command shell for the CLI onboarding flow (WEBXP-987) — the first slice of the CLI Onboarding (Phases 1–3) project.

• Registers a new top-level circleci init command (single-verb, no subcommands).
• Inside a git working tree, prints the four-phase preamble (scan → docker test → generate config → sign up) and stub [1/3] / [2/3] / [3/3] step lines that mirror the Figma prototype copy. Exits 0.
• Outside a git repo, returns a structured ExitBadArguments (exit 2) error with a cd <path> suggestion.
• Adds a permissive gitremote.InsideWorkTree() helper. gitremote.Detect() is too strict — it requires a parseable CircleCI remote that fresh init targets may not have yet.
• Real scan / test / generate / signup logic is owned by follow-up tickets and intentionally not inlined here.

Local testing

# Build
task build

# Help text — lists all four phases
./dist/circleci init --help

# Happy path — exit 0, prints welcome + 3 stub step lines + signup CTA
mkdir /tmp/init-smoke && cd /tmp/init-smoke && git init
/path/to/dist/circleci init
echo "exit=$?"   # → 0

# Error path — exit 2 with `cd <path>` suggestion
mkdir /tmp/init-no-git && cd /tmp/init-no-git
/path/to/dist/circleci init
echo "exit=$?"   # → 2

# Or install to ~/.local/bin and run as `circleci init`
task dev-install
cd path/to/any/git/repo && circleci init

Test plan

• task test passes (495 tests, includes new unit + acceptance suites)
• task check is clean (lint, license headers, go mod tidy)
• circleci init --help lists all four onboarding phases
• circleci init inside a git repo exits 0 and prints the welcome banner, [1/3] / [2/3] / [3/3] stub lines, and the signup CTA with the signup URL
• circleci init outside a git repo exits 2 and prints the "valid git repository" error with the cd <path> suggestion
• circleci init foo rejects extra args (cobra.NoArgs) with exit 2

Out of scope (per WEBXP-987)

• Real scan / test / config-generation / CIAM signup logic — each follow-up ticket
• Interactive Enter-to-continue prompts, spinners, browser open
• Telemetry hooks, localization
• --json output (not a data-returning command)

🤖 Generated with Claude Code

[Klaney]

Help output

Init in Empty Repo

Init in git repo

[Klaney]

Putting this into draft mode for now. Gonna work on the other commands first. And compose them together later if need be.