fix(lint): allow underscores in custom_block .md filenames

[rossrdme]

Problem

lint flags custom_block .md snippet files whose names contain underscores, e.g.:

[warning] custom_blocks/Helm_v4_error.md: Bad filename: should be PascalCase —
          rename to "HelmV4Error.md" so it can be used as <HelmV4Error />

This is a false positive. The filename becomes a component tag <Name />, and the only real constraint is that the name be a valid JS identifier starting with an uppercase letter. Underscores are valid identifier characters — <Helm_v4_error /> is a legal tag. The resolver already agrees: getKnownComponentNames registers the raw slug (names.add(slug)) as a known component name, so <Helm_v4_error /> already resolves. Only the filename check was overzealous, conflating "unusable as a tag" with "not my preferred style."

Fix

Flag a filename only when it can't be a component tag — i.e. it's not ^[A-Z][A-Za-z0-9_$]*$ (hyphens, or a lowercase initial). Underscores and digits are allowed. When it is invalid, still suggest the collapsed PascalCase form.

Verification

• Added unit tests: underscore name allowed; hyphenated name flagged; lowercase-initial name flagged. Full suite: 89/89 passing.
• Ran the patched linter against a real 898-page ReadMe docs repo (Cast AI): the previous Helm_v4_error.md "Bad filename" warning is gone, with no new warnings introduced.

Reported via a customer lint run (CX-3425-adjacent). Independent of #33 (Windows path fix).

🤖 Generated with Claude Code

[coderabbitai]

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