#20144 cmux NEXT: retire tabs.hiddenButtons; the pane's "…" lists every action

Summary

Since the pane's hover "…" replaced its buttons (#20119), nothing reads tabs.hiddenButtons (the action ids right-clicked off the old row). This retires it the way tabs.newTabKind was retired:

• SettingsSchema.retiredKeys lists it. An old cmux.json still loads, with no diagnostic, and settings.set answers that it was removed.
• Removed: PaneToolbarSetting, its schema row and parse, the agent-settable entry, and the two catalog strings.
• Regenerated on the merged head: the settings schema, the MDM exports (docs/mdm/*) and the settings page strings. These changes only remove entries. Base's exports still listed tabs.newTabKind, which #20127 retired in Swift, so those rows go too.

Verification

PaneToolbarHiddenButtonsRetiredTests (fleet run-swift-testing-suites.sh):

• Red on 62c8440fe6c6: all 4 expectations failed:
  • a diagnostic was raised at tabs.hiddenButtons;
  • retiredKeys was [];
  • Settings still had the row;
  • isRetired was false.
• Green on e4ae910bd399: the full package suite and a Release compile on that head (receipts in a comment).
• Exports: fleet export-settings-artifacts.sh on the merged head, then bun run strings:settings. Compared with base, the only keys removed are tabs.hiddenButtons and tabs.newTabKind, and nothing is added.
• Review: subagent review found no Swift issues. Its one finding: cmux-tui's hand-copied retired-key table (cmux-tui/crates/cmux-config/src/schema/retired.rs) lacks this key and tabs.newTabKind. That follows in a separate Rust PR.

Changelog

Removed: The tabs.hiddenButtons setting. The pane's "…" menu lists every action.

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