fix(desktop): refresh managed agent presence guidance

[mrlong0129]

Summary

Managed agents are still told that buzz users set-presence is broken, although #876 moved ephemeral publishing to authenticated WebSocket connections. Update the embedded skill and CLI testing note to describe the current behavior.

Bump NEST_SKILL_VERSION from 5 to 6 so existing managed-agent installations receive the corrected skill, and add a regression test that upgrades an installed version-5 skill through ensure_nest_at.

Related issue

Fixes #7641. The underlying CLI transport was already fixed in #876; this change updates its guidance and installation refresh behavior. Searched open PRs for #7641 and presence-skill corrections; no duplicate fix found.

Testing

• Regression reproduced before the fix: an installed version-5 skill retained the stale warning.
• After the fix, the new upgrade regression and all 28 managed_agents::nest::tests pass.
• git diff --check passes.
• just ci was attempted. Formatting, lint/static checks, and multiple Rust unit suites passed. The first buzz-acp run had two failures (claude_named_adapter_wire_lifecycle_records_prompt_and_cost and pi_launch_preserves_existing_skills_in_explicit_workspace); its subsequent run passed with 934 tests.
• Full CI is incomplete: stopped during relay channel-authorization compilation when local disk usage reached 97%. The later desktop/build/mobile test stages were not completed by this run. Keeping this PR as a draft pending complete CI.

No desktop UI was changed.

[github-actions]

🔐 Codex Security Review

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