feat(cli): autosdk skill emits multifile bundle by default

Split the SKILL.md output into three complementary files so the main
manifest stays agent-loadable even for huge specs like OpenAI's 241-op
surface:

  SKILL.md     — concise groups-only overview (3.6 KB for OpenAI)
  commands.md  — full per-command reference with HTTP routes
  auth.md      — credential resolution order + detected auth schemes

SKILL.md frontmatter + description format unchanged. It now points
agents at the companion files via relative markdown links and at the
CLI's own `skill <name>` subcommand for on-demand retrieval.

`--output` is now a directory by default. Pass `--single-file` (or an
`--output` path ending in `.md`) to get the monolithic output instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: HavenDV