feat: paste expand toggle - #41813

A pasted [Pasted ~N lines] placeholder in the prompt can now be
expanded to its full content and collapsed back -- click it, or run
"Expand/collapse pasted text" (prompt.paste.toggle) from the palette
or a keybind.

Ported from upstream anomalyco#8501 (325 reactions -- the
highest of any issue found across a full survey of this repo's open
issues). A trivial 1-file fix already merged upstream as anomalyco#39920 isn't
even on this fork's history and is a one-way, no-collapse expand
anyway, not the real ask. Richer attempts were all closed by the
automated PR-cleanup bot and never reviewed:

- PR anomalyco#41813 "paste expand toggle" is the primary source: a pasted-
  text part remembers its original placeholder so it can collapse
  again, the chip is swapped by deleting its extmark, replacing the
  selected text, and recreating the extmark over the new text.
- PR anomalyco#33700 fixed a real, separate bug reused as-is:
  expandPastedTextPlaceholders substituted text back in via
  String.replace(str, str2), which treats $&/$$/etc. in the pasted
  text as special replacement patterns and silently corrupts it --
  fixed by passing a replacer function instead of a literal string.
- PR anomalyco#37052 contributed only the idea of a palette command to expand
  chips (its own version was one-way, expand-all); this fork's
  version is a proper per-chip toggle.

New: older saved prompts with no stored placeholder can still be
expanded (anomalyco#41813's version would lock them collapsed-only). Two pure
functions (togglePastedText, replaceVirtualExtmarkText) were extracted
into packages/tui/src/prompt/part.ts so anomalyco#41813's logic is unit-
testable outside the full Prompt component.

Known gaps: the full Prompt component's mouse-click path has no
automated test (no existing fixture mounts it with SDK/sync/route
providers -- checked manually against a bare textarea instead);
clicking a chip's very last character can miss the same way it does
in anomalyco#41813; an expanded chip is viewable but not editable in place.

New tests in packages/tui/test/prompt/part.test.ts and a new
packages/tui/test/prompt/paste-toggle.test.tsx (using this repo's
existing testRender/mockMouse helpers). Verified against this fork's
current dev (not just the agent's own stale worktree base): bun
typecheck clean; full packages/tui suite 215 pass.

Co-Authored-By: Claude Sonnet 5 <[email protected]>