docs(sdk): document launch-time skill overlays

[kyletser]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Document agent_launch_additions.skills_append in the Agent Server architecture guide.

• Provide a request example that validates against the current SDK model.
• Explain same-name precedence, disabled_skills, persistence across server restarts, and disable_model_invocation for trigger-only skills.
• Clarify that launch additions change the materialized conversation agent without modifying the stored Agent Profile.

Validation:

• The JSON example passed StartConversationRequest.model_validate against the matching SDK branch.
• git diff --check passed.
• mint broken-links parsed the updated page. The repository-wide check still reports 928 existing broken links; this section adds no links.

Implementation: OpenHands/software-agent-sdk#4717

Related to OpenHands/OpenHands#16905.

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