Init: make documented --builders selection work from a fresh repository

[jeffhuber]

Problem

Documentation and generated guidance still show code-mower init --builders codex,claude,cursor, but that command exits non-zero in a fresh repository without code-mower.yml. The equivalent command works only with --easy or --with, so the documented first-run path contradicts the CLI contract.

Outcome

A newcomer can copy every documented builder-selection command in a repository with no Code Mower configuration and receive a clear preview before any mutation.

Acceptance criteria

• Choose and enforce one cold-start contract: either make bare init --builders ... use the starter configuration, or replace every first-run occurrence with the required --easy/--with form.
• Prefer one canonical spelling across help, docs, session guidance, templates, and generated next steps.
• Preview remains the default; writing files still requires explicit apply/output behavior.
• Existing repositories continue to use their selected configuration and profile without silently falling back to the starter.
• Error messages distinguish a missing configuration from an invalid requested lane.
• Add fresh-directory, configured-repository, profile, dry-run, and documentation-contract tests.
• Deliver as one independently reviewed PR after Devin v1.4.0; it does not block Release: publish and qualify Code Mower v1.4.0 with Devin peer support #912.

Evidence

Reproduced in a metadata-only v1.3.1 adoption run on a fresh hosted checkout. init --easy --builders ... and init --with ... worked; the documented bare form failed before preview.