fix(skills): add normalize_skill_files() helper and document path naming convention

[ppradyoth]

Summary

Closes #1575.

beta.skills.create(files=...) silently requires every path to be prefixed
with a top-level directory whose name exactly matches the name: field in
SKILL.md frontmatter (e.g. my-skill/SKILL.md, my-skill/scripts/tool.py).
When callers pass bare names like "SKILL.md" they receive a cryptic 400 with
no guidance on what went wrong.

What this PR does

Normalization is now automatic — Skills.create / AsyncSkills.create
call normalize_skill_files() internally before building the multipart request,
so callers never need to know about the prefix requirement:

# All three are now equivalent — the SDK fixes the paths transparently:
client.beta.skills.create(files=[("SKILL.md",        skill_md, "text/markdown")])
client.beta.skills.create(files=[("wrong/SKILL.md",  skill_md, "text/markdown")])
client.beta.skills.create(files=[("my-skill/SKILL.md", skill_md, "text/markdown")])

This makes upload symmetric with _archive_top_dir on the download side, which
already strips the wrapper directory when extracting — the constraint was always
there in both directions, but only enforced on upload.

Name resolution order

1. name: field in SKILL.md YAML frontmatter (primary)
2. display_title normalised to lowercase-with-hyphens (fallback when frontmatter omits name:)
3. If neither is available the original file list is passed through unchanged and the API will surface a descriptive error

Key advantages over the other PRs addressing #1575

	This PR (#1590)	#1577 (docs only)	#1604 (auto-wire)
Auto-wired into create()	✅	❌	✅
Wrong prefix replaced (not double-prefixed)	✅	—	❌ bug
display_title fallback	✅	—	✅
Idempotent (apply twice → same result)	✅	—	✅
PathLike file content	✅	—	❌
normalize_skill_files() exported for explicit use	✅	—	❌

Changes

• src/anthropic/lib/tools/_skills.py — normalize_skill_files(files, *, display_title=None) with wrong-prefix replacement, display_title fallback, and idempotency; _parse_skill_name_from_frontmatter returns str | None instead of raising
• src/anthropic/lib/tools/__init__.py — re-exports normalize_skill_files for callers who want to inspect normalized paths before uploading
• src/anthropic/resources/beta/skills/skills.py — auto-wires normalization into both Skills.create and AsyncSkills.create; updated files= docstring to say normalization is automatic
• tests/lib/tools/test_skills.py — 27 tests total (10 new: idempotency, display_title fallback, special-char normalization, 2-tuple entries, PathLike content, frontmatter returning None, wrong-prefix replacement)

Test plan

uv run pytest tests/lib/tools/test_skills.py -v  # 27 passed