docs: document MCP secret resolution fallback behavior

Adds documentation explaining how the CLI resolves secrets for MCP servers,
including the fallback from org/package secrets to local environment variables.
This documents the fix from PR #8899 that allows env vars to override
inaccessible org secrets in devboxes and restricted environments.

Co-authored-by: nate <nate@continue.dev>

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Co-Authored-By: Continue <noreply@continue.dev>