feat(zsh): transient prompt support

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As of v0.3.0 (c73dc6c).

Summary

Transient prompt — collapsing previous prompts to a minimal one-liner (typically just the prompt character) after a command is accepted — is the most-requested comfort feature in the pure/powerlevel10k family and fits Nova's philosophy directly: scrollback stays dense, and the full two-line prompt only occupies the active line.

Nova's architecture makes this cheap: the zsh adapter already owns PROMPT/RPROMPT and has a zle integration point; no worker round trip is needed for the collapsed form.

Proposed design

• Config: a [transient] table (or layout.transient) with enabled and an optional segment list defaulting to ["prompt_char"].
• zsh side: in a zle-line-finish hook, swap PROMPT to the pre-lowered transient string and zle reset-prompt; restore the full prompt in precmd (which already re-renders every prompt).
• The worker can pre-lower the transient prompt once per config generation and ship it in the handshake (or alongside each Prompt record) so the swap is a pure variable assignment — zero IPC on the hot path.
• Interaction to specify: multi-line input, Ctrl-C aborts, and RPROMPT clearing (RPROMPT='' in the transient state is the conventional choice).

Notes

• This pairs with the keymap-select fix (fix(zsh): vi_command prompt character never appears (missing zle-keymap-select hook) #26): both need add-zle-hook-widget wiring beyond line-init, so implementing them together amortizes the zle plumbing.
• Automated coverage depends on the harness tracked in test(zsh): add an interactive zsh E2E harness (zpty) #24.

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