Fix --from-template: preserve frontmatter and copy sibling assets (#82, #83)

[devrimcavusoglu]

Summary

• skill create --from-template does not preserve frontmatter fields (description, metadata) #82 — skill create --from-template <SKILL.md> reset description, tags, metadata.author, and metadata.version to placeholder defaults because the file was read opaquely into the body, producing a malformed manifest with two stacked frontmatter blocks (only the default block was visible to the parser).
• skill create does not copy sibling assets (references/, templates/, VENDORED.md) from template directory #83 — --from-template only ever read a single file, so sibling assets (references/, templates/, VENDORED.md, …) were silently dropped.

Both issues share a single root cause: the flag was implemented as "read one file as raw body" while users (e.g., AURA's init flow) reasonably treat a skill as a folder. This PR makes --from-template accept all three sensible inputs and behave correctly for each.

Behavior

--from-template <path> now resolves three ways:

<path> is	Behavior
Directory containing SKILL.md	Parses frontmatter; recursively copies every sibling file/subdir alongside the new SKILL.md.
SKILL.md file (starts with ---)	Parses frontmatter. (No siblings — point at the directory for those.)
Any other markdown file	Contents become the new skill's body verbatim (legacy behavior preserved for backward compatibility).

In the manifest-aware modes, the CLI <name> always wins over the template's name. Other CLI flags (--description, --tags, --author*, --version) override template values only when explicitly set (via cobra.Flags().Changed()), so an unspecified --description no longer clobbers the template's description with the Use when TODO: placeholder.

Implementation

• New internal/registry/copy.go::CopySiblings(srcDir, dstDir) — recursive copy that skips the top-level SKILL.md. Lives in registry/ per the layering note in AGENTS.md ("filesystem operations belong here, not in cli/").
• internal/cli/skill_create.go — loadTemplate resolves the path; buildSkillFromTemplate merges template + CLI flags. Falls back to the existing NewSkillWithBody path when no template is given.
• Registry.Create signature unchanged — sibling copying is a follow-on step in the CLI command, with rollback (os.RemoveAll(path)) if the copy fails.

Test plan

• Unit: TestCopySiblings_* — copy nested files, skip top-level SKILL.md, copy a nested SKILL.md, handle empty dir.
• CLI regression for skill create --from-template does not preserve frontmatter fields (description, metadata) #82: TestSkillCreate_FromTemplate_SkillMdFile_PreservesFrontmatter — description, tags, version, author all preserved; only name overridden.
• CLI regression for skill create does not copy sibling assets (references/, templates/, VENDORED.md) from template directory #83: TestSkillCreate_FromTemplate_Directory_CopiesSiblings — verifies references/architecture.md, templates/example.txt, VENDORED.md land in the new skill with byte-identical content.
• CLI: TestSkillCreate_FromTemplate_CLIOverridesTemplate — --description and --tags override; unspecified --author/--version inherit from template.
• CLI: TestSkillCreate_FromTemplate_RawBodyFile — legacy raw-body path still works; produces exactly one frontmatter block.
• CLI: TestSkillCreate_FromTemplate_DirectoryMissingManifest — clear error when the directory has no SKILL.md.
• make fmt clean, make lint 0 issues, make test all green.
• Manual end-to-end against the rebuilt binary using a source skill with references/, templates/, and VENDORED.md — all assets copied, frontmatter preserved, single clean frontmatter block in the output SKILL.md.

Closes #82
Closes #83

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