feat(tui): open a paste placeholder's text by clicking it, to edit or inline - #54463

… text

input_undo ran the textarea's own history, which steps back one
character at a time, so taking back a paragraph took dozens of
presses. It knew nothing of the prompt's parts: undoing the deletion
of a "[Pasted ~N lines]" placeholder brought its label back as plain
text with the pasted content gone. A submit cleared it, so text lost to
a stray submit or ctrl+u could only be found again through the
history.

The prompt now keeps its own history (src/prompt/undo.ts) and binds
input.undo and input.redo to it while focused. Typing is grouped by
word: a group ends where a new word starts, after a pause over a
second, when the caret moves, or between typing and deleting. A paste,
a word or line delete, a newline, clearing, recalling history and a
submit are each one step. Every entry records the text, the parts with
their pasted content, the mode and the caret, and undo restores them
together. The history lives outside the prompt component, so after a
submit undo brings back what was sent, even once the route has moved
to the new session, without touching the running turn.

prompt.undo_depth sets how many steps are kept (default 500). The
default keys are unchanged; keybinds.mdx shows how to move suspend off
ctrl+z and use ctrl+z / ctrl+shift+z.

…der says

A paste of three lines or more, or over 150 characters, always became
"[Pasted ~N lines]", which says nothing about what was pasted; only the
global toggle could change it.

prompt.paste_summary in tui.json sets the line and character
thresholds and the placeholder format, with {lines}, {characters},
{bytes} and an {excerpt} of the paste's start, whose length is
configurable. pasteSummary (src/prompt/paste.ts) builds it and keeps it
on one line. The defaults reproduce the previous rule and label
exactly; the toggle still turns summarizing off.

… and todos

Many parts of the TUI show something a keybind or slash command acts
on, but did nothing when clicked. Each now runs that command through
the keymap, or copies what it shows:

- prompt meta row: the agent, model and variant open their pickers; the
  "tab agents" and "ctrl+p commands" hints run agent.cycle and the
  command palette; "esc interrupt" runs session.interrupt, so two clicks
  interrupt like two esc presses;
- usage below the prompt and the sidebar Context lines open /status;
- the directory below the prompt and in the sidebar and home footers is
  copied;
- the model in an assistant footer opens the model picker on the model
  that turn ran on (DialogModel takes a current model);
- the sidebar title renames the session; an MCP row connects or
  disconnects that server without opening /mcps (the toggle, status
  refresh and in-flight guard move from DialogMcp into local.mcp); a
  Modified Files row opens /diff at that file (diff route `file`); a
  todo is copied and appended to the prompt;
- a session or message ID (ses_, msg_) is copied wherever it is drawn.
  ui/id-click.ts reads it back from the cells under the pointer inside
  the clicked renderable, and message and tool click handlers stand down
  when one is under the pointer;
- the home footer's MCP count opens /status and "Connect provider"
  opens /connect.

ui/click.ts runs an action only for a left press and release on the same
cell that leaves no text selected, so drag-to-copy keeps working on all
of these elements.

… inline

Once a paste became "[Pasted ~N lines]" its text could not be seen or
changed without sending it or opening the whole prompt in $EDITOR.

Clicking a pasted-text placeholder opens DialogPaste: the text in the
prompt's own editor, a textarea with the same input keybinds, mouse
caret placement and grouped undo, where enter adds a line. Closing it
(esc, ctrl+c, a click outside) saves: the part gets the edited text,
and the label is rebuilt with pasteLabel when the text changes it, such
as its line count. Inline replaces the placeholder with the text, and
saving empty text removes it. Both are one undo step in the prompt.

The full-app prompt test harness moves to test/fixture/prompt-app.ts,
now shared by the undo and paste tests, and gains clicks and frames.

…ed by tab

The paste dialog has two buttons above its editor: Keep as block saves
the edit and keeps the placeholder, the same as enter or esc, and
Insert inline replaces the placeholder with the text. Tab moves focus
from the editor to Keep as block, Insert inline and back to the editor
wherever the caret is, and shift+tab goes the other way; dialogs bind
tab literally, as dialog-update.tsx does. Up on the editor's first row
still reaches the buttons, left and right move between them, enter
presses the focused one (it flashes first) and down returns to the
text.

The save hint moves under the editor's box at its right end, next to a
tab hint, where the prompt shows ctrl+p commands; on Insert inline it
reads "esc save", since enter would inline. The editor's room subtracts
the extra hint row, so a short paste still shows whole and the buttons
stay on screen.