feat: add internal/scaffold package for cascade init

[joshua-temple]

Problem

After the inline run: removal, cascade callbacks are reusable-workflow-only, so a cold start requires hand-authoring workflow_call files. There is no programmatic way to produce a known-good starter manifest plus the build/deploy stubs it references.

Fix

Adds a new internal/scaffold package (PR1 of cascade init, no CLI yet). It produces an in-memory set of files for a chosen topology and guarantees they are never handed out broken.

• Scaffold(project, trunkBranch, envs, ...Option) returns a path->content map: a starter .github/manifest.yaml plus the build (and, when environments exist, deploy) reusable-workflow stubs it references. An empty envs slice yields the no-env, release-only topology.
• The manifest carries the # yaml-language-server: $schema=... directive for editor autocomplete (inert YAML comment), pins cli_version to config.DefaultCLIVersion, and applies environment names positionally.
• Stub workflows are valid on: workflow_call reusable workflows declaring the environment/sha/dry_run inputs cascade passes; the build stub wires the artifact_id output through, matching the callback contract.
• SelfCheck(files) writes the files to a temp dir and confirms they parse (config.ParseManifestFile), validate (config.Validate), and generate (internal/generate orchestrate + promote) without error. Scaffold runs it before returning.
• Topology presets: no-env [], two-env [dev prod], three-env [dev staging prod], four-env [dev test uat prod].

Verification

• go build ./..., full go test ./... (all packages green), and golangci-lint run ./... pass.
• Table-driven unit tests over all four topologies: each scaffold parses, validates, and generates; stub input/output keys are asserted against the contract; the $schema directive and positional env ordering are checked; negative SelfCheck cases (invalid env name, malformed YAML, build missing workflow:) exercise real validation failures.
• New code is confined to internal/scaffold/; no dogfood-workflow drift.

PR2 will wire the cascade init cobra command on top of this package; cascade adopt will reuse the same writer and self-check.