docs: clarify DomainAgent first implementation step (deterministic DNS planning core)

[MYaelMendez]

Motivation

• Provide a clear, implementation-ready first milestone for the Sovereign Gateway onboarding agent that isolates DNS planning from network/registrar side effects.

Description

• Expanded docs/domain-agent-first-step.md with a focused “The first step to bring the agent to life” section that defines the deterministic DNS planning core and its responsibilities.
• The new guidance lists the concrete responsibilities: accept a domain + did:plc request, compute the _atproto TXT (did=did:plc:...), merge it against existing records by removing conflicting _atproto entries while preserving unrelated records, and return a reviewable before/after plan object.
• Kept the change documentation-only so the planner/agent codebase contract remains stable for subsequent tasks like OAuth transport, Namecheap API wiring, and DNS propagation polling.

Testing

• Ran PYTHONPATH=. pytest -q which passed all tests (4 passed).
• Observed pytest -q fails in this environment without PYTHONPATH because the forge package is not installed in editable mode, which does not affect the repository-level unit tests when run with PYTHONPATH=..

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Codex Task