notation: warn when a directive is not in a comment the language reads

[ewels]

Closes #1

A directive written in a comment syntax that the block's language does not read, such as <!-- [!callout …] --> in an mdx block or // [!code focus] in a Python block, stayed in the code as text with no build warning. parseLine() in notation.ts, the one place every directive goes through, now warns when a line has no comment that holds a directive but has a known, unescaped directive outside a string. The message names the syntaxes to use, for example: src/content/docs/example.md, mdx code block, line 1: [!callout /import/]is not in a comment that this block reads. Use{/* */}. It shows as text.

Which blocks warn: only languages that have a comment syntax (built-in or from notation.comments). A language with no syntax, or one set to [] (the docs site does this for md, markdown and mdx), reads no directives and never warns. Escaped directives ([\!code …]), directives in a string, and unknown [!word] text such as Markdown alerts or arr[!flag] stay silent. No new opt-out was added; the decision is in .agents/DECISIONS.md.

pnpm lint, pnpm test, pnpm lint:docs and pnpm docs:build pass, and the docs build logs no warnings.

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