Agent Sessions: make Additional SSH hosts visibly editable and improve remote diagnostics

## Summary The **Settings → Menu → Agent sessions → Additional SSH hosts** row does not present a visible editing affordance in the grouped macOS form. It renders like a static label rather than an editable setting: there is no visible field boundary, placeholder, example, or add action. This report is based on the public implementation and intentionally contains no user configuration or environment details. ## Current implementation ```swift TextField(L("agent_sessions_hosts_title"), text: self.$settings.agentSessionsManualHosts) .disabled(!self.settings.agentSessionsEnabled) ``` In this form layout, the localized title is visible while the empty editing area is not visually discoverable. A user can reasonably interpret the row as inert text. The public documentation describes the value as a comma-separated list of SSH destinations such as `user@host`, but the settings UI does not expose that format or the remote prerequisites in context. ## Expected UI At minimum, render an explicit labeled editor: ```swift LabeledContent(L("agent_sessions_hosts_title")) { TextField("user@host, user@host", text: self.$settings.agentSessionsManualHosts) .labelsHidden() .textFieldStyle(.roundedBorder) .frame(minWidth: 240) } .disabled(!self.settings.agentSessionsEnabled) ``` A structured editor would be clearer than a comma-separated string: - one row per SSH destination; - visible **Add** and **Remove** controls; - separate host/SSH-user fields, or a clearly labeled `user@host` field; - optional port/SSH-alias support through normal SSH configuration; - a **Test Connection** action; - inline status and a useful accessibility label/help description. ## Remote diagnostic UX The menu currently reduces remote failures to broad states such as `unreachable` or a generic timeout. The app should preserve privacy while distinguishing actionable failure classes: - peer offline or hostname resolution failure; - TCP/SSH connection timeout; - SSH authorization rejected; - interactive authentication required despite batch mode; - remote `codexbar` missing from the non-interactive `PATH`; - incompatible remote CLI/protocol; - remote session scan failure. The UI need not expose raw command output. A sanitized, categorized message plus a troubleshooting disclosure would be sufficient. The settings section should also explain the remote contract in generic terms: remote hosts require non-interactive SSH and a compatible CodexBar CLI available to the remote shell. ## Acceptance criteria - The empty Additional SSH hosts control is visibly editable in light and dark appearances. - The accepted input format is shown in the UI. - Keyboard focus and VoiceOver identify the control as editable. - Users can test a configured destination without waiting for periodic refresh. - Common remote failures have distinct, sanitized guidance. - Tests cover empty/non-empty layout, disabled state, keyboard/accessibility behavior, and representative error classification. - Relevant localized strings and user documentation are updated. ## Scope This issue concerns remote-session setup and diagnostic usability. Destructive session lifecycle actions, such as stopping or terminating an agent process, should be designed and reviewed separately.