chore: remove dead Swift code, unused strings and stale comments

[arzafran]

What this does

Removes about 1,300 lines of Swift code that nothing calls, 18 UI strings nothing shows, and comments that describe old history instead of the current code. The app behaves the same. The four tmux-compat commands that only ever printed "not supported yet" (popup, bind-key, unbind-key, copy-mode) now report an unknown command instead.

Summary

Review order:

1. Sources/TerminalController.swift, Sources/TerminalController+Debug.swift, CLI/programa.swift, CLI/SocketClient.swift: zero-caller helpers and constants. The at-budget files gain headroom.
2. contracts/v2/methods.json and the generated files: debug.viewtree removed (no caller in CLI, MCP, tests or scripts). Regenerated with gen-v2-contract.py.
3. Sources/WorkspaceContentView.swift, Sources/Panels/TerminalPanel.swift: the tmuxActivePane overlay target, which never had a producer for its layout report. Documented in docs/removed/tmux-active-pane-overlay.md. The overlay settings gate and the bonsplitPane target stay, since live code and tests use them.
4. TabManager, Workspace, model and UI members with no callers; 20 unused shortcut wrappers in KeyboardShortcutSettings.swift; promptRenameSelectedWorkspace; the empty Sources/GhosttyTerminalView.swift and its project entries.
5. The rest is Localizable.xcstrings deletions (each key had 0 references) and present-tense rewording of history comments.

Contract change: tests_v2/test_tmux_compat_matrix.py now asserts the four removed tmux commands are rejected, instead of asserting the old "not supported" message.

Kept on purpose: recordSurfaceDraw and the draw-count stats (a UI test reads renderDrawCount), and programaWithWindowFirstResponderBypass (per-event path, needs its own review).

Test plan

• check-structural-budgets.py, check-v2-contract.sh, check-settings-docs.py pass locally.
• programa, programa-cli and programa-unit build-for-testing succeed locally.
• CI unit, UI and socket suites pass (the UI test target was not compiled locally).