feat(agentic-server): describe single-provider options as a convenience, not legacy

[pyramation]

Summary

This is a comment-only change that carries out the agentic env-var decision on constructive-io/constructive-planning#2152. The single-provider options (providerType, providerBaseUrl, providerApiKey, defaultModel) stay as the one-provider dev convenience, but they are no longer labelled "Legacy". Behaviour does not change.

Layer

• Layer: agentic/agentic-server options docs.
• What is removed: the "Legacy" labels on the single-provider options and on the single-provider fallback in resolveProvider.
• Why it was compat-only: the options are current behaviour. They were only labelled as if they were kept for older callers.

Link to Devin session: https://app.devin.ai/sessions/0f9216183d904190b25c5344962e2f12
Open in Devin Desktop: https://app.devin.ai/desktop/session/0f9216183d904190b25c5344962e2f12?variant=devin
Requested by: @pyramation

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[tenki-reviewer]

Review complete. No issues found — approved ✅.

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This PR touches only comment content in the agentic server providers and types files, with no changes to executable logic.

_{Reviewed commit: e981c82}