[Agent Builder] Update access control docs: new agents default to Private

[charlotte-hoblik]

Summary

Closes https://github.com/elastic/docs-content-internal/issues/1609

custom-agents.md said new agents default to Public. kibana#282015 (9.6) changed that to Private. Verifying it turned up three related changes across two minors, so this PR covers the whole access control section rather than one line:

• Default for new agents is now Private, badged 9.6+. The old Public default is scoped 9.4-9.5. Existing agents and built-in agents are unaffected and stay Public, which is worth stating explicitly.
• The setting was renamed from Visibility to Access control in 9.5 (kibana#273500). Renamed the step, the section, and the glossary entry, with a 9.4-scoped note for readers still on the old label. Heading anchor redirect added.
• Who can change the level is documented, since the control is read-only for anyone who is not the owner, an administrator, or a Manager grantee.
• chat.md: the agent list badge list omitted Private. Also clarified that Read-only marks built-in agents rather than being an access level.
• kibana-api.md: noted that omitting access_control on create produces a private agent.

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🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/chat.md
• explore-analyze/ai-features/agent-builder/custom-agents.md
• explore-analyze/ai-features/agent-builder/glossary.md
• explore-analyze/ai-features/agent-builder/kibana-api.md

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