fix: add structured help JSON and provider BASE_URL validation

#683-#692: help topic JSON now includes usage, purpose, formats,
related, local_only, requires_credentials, and aliases extracted
from help prose. Export and doctor keep their custom structured
responses.

#466: new check_base_url_health() validates ANTHROPIC_BASE_URL,
OPENAI_BASE_URL, XAI_BASE_URL, and DASHSCOPE_BASE_URL for basic
HTTP(S) URL format. Non-http schemes and empty values produce a
warn-level diagnostic.

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Author: 2 people