feat(skill): add include file policies

[samzong]

Summary

• Add object-form skill.include support for per-file append, create, replace, and omit policies.
• Render generated Skill output through an in-memory file plan so omitted module references are removed before SKILL.md is written.
• Document the include policy behavior and remove obsolete include-specific tests.

Verification

• git diff --cached --check
• go test ./internal/codegen/render -count=1
• go test ./internal/lathecmd -count=1
• go test ./... -count=1
• make check

Compatibility

Existing skill.include: <path> configuration remains supported with the default append/create behavior. The new object form only changes generated Skill output policy. No runtime catalog schema, auth, body, output, or generated command shape changes.

Checklist

• Tests or focused verification cover the changed surface.
• User-facing behavior changes are documented.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.