Add on-demand and hidden input window display modes

[SayreBlades]

Summary

Adds a pi-coding-agent-input-window-display user option so the input window can be ephemeral rather than always-on, enabling a chat-centric workflow where the input pane appears only while composing.

• always (default, unchanged): the input window is shown whenever the session is displayed.
• on-demand: the input window is shown at launch, hidden after each send, and reopened with the new M-x pi-coding-agent-open-input; pi-coding-agent-toggle / redisplay show chat-only.
• hidden: like on-demand, but a session also launches chat-only — the input never appears until you summon it (i/a under Evil, or M-x pi-coding-agent-open-input).

Motivation

Today the input pane is permanent chrome even when you're only reading agent output (scrolling tool output, following streaming thinking, reviewing a diff). On a small laptop that's a meaningful chunk of the chat you're actually trying to read, and the input buffer is useful only for the few seconds you spend composing a prompt.

The only current recourse for a chat-centric view is fragile user advice:

(defadvice! my/pi-send-close-input :after #'pi-coding-agent-send
  ;; delete the input window after sending
  ...)

That advice is incomplete in two ways:

1. It can't make launch chat-only. Launch always goes show-session-buffers → display-buffers, which unconditionally splits. So after the advice closes the input on send, the next focus/launch re-opens the input split — the user closes it by hand over and over.
2. It duplicates the package's window-management internals (soft-dedication of the input window, frame-local window queries, re-selecting the chat window, the window-parent guard) and silently breaks when those internals move.

Additionally, non-Evil users have no way to dismiss the input at all today — only the Evil integration has pi-coding-agent-evil-close-input (q).

This PR makes the input a first-class transient pane — completing the existing dismissable-input design (Evil i/a to focus, q to close) — and lets users delete their hand-rolled advice in favor of one variable.

Changes

• New option pi-coding-agent-input-window-display (always / on-demand / hidden).
• New command pi-coding-agent-open-input: focus an existing input window, else split one below the chat window, else restore the full layout.
• pi-coding-agent--display-buffers gains an optional chat-only argument; the input-splitting logic is extracted into a pi-coding-agent--split-input-below-chat helper (reused by open-input).
• New predicate pi-coding-agent--input-window-on-demand-p groups the two ephemeral modes (on-demand and hidden) for shared behavior: hide after send, and chat-only on redisplay/toggle.
• pi-coding-agent--show-session-buffers honors hidden (chat-only launch). Note: when both windows are already visible, the existing layout is preserved and input is focused — it never yanks an intentionally-opened input away.
• Hide-after-send wired into pi-coding-agent-send via pi-coding-agent--maybe-hide-input-window.
• Bugfix: pi-coding-agent--best-input-window could return a non-input window when no input window was visible; it now returns nil. Under on-demand/hidden, "no input window visible" is a normal state rather than an error state, so this becomes load-bearing.

Design notes

• One enum, not two booleans. Parallel to pi-coding-agent-thinking-display and pi-coding-agent-project-trust-policy (both symbol enums). hidden reuses on-demand's ephemeral behavior; the only delta is the launch window, which is the dedicated knob this exposes. A cross-product of booleans (hide-after-send × chat-only-at-launch) would be harder to read and extend (side-by-side / input-only layouts later).
• Naming/values are negotiable — happy to adjust to maintainer preference.

Compatibility & testing

• Default is always — no behavior change for existing users.
• pi-coding-agent--display-buffers's new arg is optional and backward compatible.
• Six unit tests added/updated in pi-coding-agent-ui-test.el (batch-runnable via make test-ui), all passing:
  • chat-only launch under hidden
  • send-hides under on-demand and under hidden
  • send-keeps under always
  • open-input splits-below-chat and focuses-existing-input
  • display-buffers chat-only
• Byte-compiles clean with byte-compile-error-on-warn; checkdoc clean on changed source files.

Window-layout logic is unit-covered; make test-gui is unaffected by this change.

[dnouri]

Sorry but this is outright bullshit. The author of this PR has not understood what this project is about at all. But thanks I guess.