fix: return structured JSON from docs ask/chat when --json is active

[ssh-esh]

Closes #160

When --json is passed, docs ask and docs chat now buffer all streamed chunks
and return { "answer": "..." } instead of streaming markdown to stdout and
appending null.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	2.77s
✅ REPOSITORY	gitleaks	yes		no	no	8.25s
✅ REPOSITORY	git_diff	yes		no	no	0.03s
✅ REPOSITORY	secretlint	yes		no	no	2.1s
✅ REPOSITORY	trivy	yes		no	no	13.88s
✅ TYPESCRIPT	eslint	4		0	0	2.51s

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[JoshMock]

A good approach for now.

If there was a way to separate the input stream into chunks by sentence, paragraph or section, each chunk could be streamed to stdout as one JSON object per line. That way an agent could choose to kill the NDJSON stream early once it has its answer, rather than wasting tokens on content it doesn't need. That's likely a much more complex solution, though, so this works. 👍

[MattDevy]

LGTM