WorkspaceBuilder: make pane readiness configurable

[tony]

Summary

Make pane readiness behavior configurable independently from builder selection, so users can choose compatibility or speed without swapping the entire workspace builder.

Related: #1053, #1056, #1058.

Proposed behavior

• Add a pane_readiness configuration policy that can be used by the classic builder and future builders.
• Preserve the current safe behavior by default unless a compatibility decision changes it deliberately.
• Support an auto mode that enables readiness waiting where tmuxp knows it matters, especially zsh prompt redraw safety.
• Support an always mode for users who want the safest prompt/layout behavior.
• Support a never mode for users who prefer speed and accept prompt/layout race risk.
• Keep this policy separate from parallel builder work so users can tune readiness without opting into relaxed build ordering.

Example shape:

pane_readiness: auto

Acceptance criteria

• The default preserves existing workspace behavior.
• Tests cover auto, always, and never behavior for default-shell panes and custom shell/window shell cases.
• Documentation explains the zsh prompt-readiness motivation and the speed/compatibility tradeoff.
• The option remains usable with the classic builder even before a parallel builder exists.

[tony]

Shipped in v1.73.0 (#1066). A workspace_builder_options.pane_readiness policy (auto | always | never, plus truthy/falsy aliases) controls the pane-prompt wait — exactly the shape proposed in #1058. The new auto default waits only when the session shell is zsh (the shell the wait was added for), so non-zsh workspaces skip it; always restores wait-everywhere and never skips it entirely.