feat: add cascade init command to scaffold a project

[joshua-temple]

Problem

cascade had a scaffold package but no command to drive it. New adopters had to hand-write the manifest and callback stubs before they could generate anything.

Fix

Adds cascade init, the command that drives internal/scaffold.

Flags: --topology (preset: no-env/two-env/three-env/four-env, default two-env), --envs (custom ordered list, overrides --topology, last name is the release stage), --name (default: target dir base name), --dir (default .), --cli-version, --force, --dry-run.

Behavior:

• Resolves the environment list, then calls scaffold.Scaffold, which self-checks (parse + validate + generate) before returning. A scaffold that cannot survive the real generator is never written, so validate-failure rollback is inherent.
• Refuse-on-existing: if any target file exists and --force is unset, aborts and lists the conflicts, writing nothing. --force overwrites.
• --dry-run: prints the file list and exits without writing.
• On success: writes the files and prints a summary plus next steps (fill in the stubs, commit, run generate-workflow), noting the manifest already carries the $schema directive.

Docs

• cli-reference: new init entry with the full flag table.
• getting-started: a "Fast path: scaffold with cascade init" section.
• adoption: points at cascade init as the fast path for building a pipeline from scratch.

Housekeeping

go mod tidy flips santhosh-tekuri/jsonschema/v6 from indirect to direct (it is a direct test dependency) and records its transitive go.sum entries. One-line marker fix; nothing else pulled in.

Verification

• go build ./..., go test ./... (1345 pass), golangci-lint run ./... (clean).
• Unit tests cover topology resolution, custom --envs, refuse-on-existing (with and without --force), --dry-run writes nothing, a successful init whose written manifest re-passes self-check, and the scaffold-failure path.
• Dogfood drift: generate-workflow --config .github/manifest.yaml --force then git diff --exit-code -- .github/workflows .github/actions is clean.
• Manual run of cascade init --topology two-env --name demo produces a valid scaffold that generate-workflow --validate-only accepts.