Deprecate source and introduce agent as mcp connected to agents, agent makes more sense than source

Proposal: Deprecate the use of 'source' and introduce 'agent' as the canonical term for MCP-connected entities. Rationale: The term 'agent' more accurately reflects the architectural role and future extensibility of these components compared to 'source'.

• Update all references of 'source' to 'agent' across the codebase, documentation, and integration APIs.
• Ensure all MCP connections and orchestration flows reference 'agent'.
• Update migration and onboarding flows to use 'agent' terminology exclusively.

Context: This aligns with ongoing terminology standardization and will improve clarity for all users and developers working with MCP and agent orchestration.

Please triage, assign, and label as appropriate for architecture/API refactor.