Add a keyboard shortcut for toggling thinking mode

[LarsArtmann]

TL;DR: enabling/disabling thinking is only reachable through the command
palette (ctrl+p → type → enter); it has no key binding anywhere in the
keymap. I'd like a direct shortcut, wired the way ctrl+b toggles the
sidebar.

Problem

For models with CanReason and no ReasoningLevels (Anthropic-style),
the only way to flip thinking is the command palette:

• internal/ui/dialog/commands.go:475 registers the command with an
  empty shortcut hint: NewCommandItem(..., "toggle_thinking", status+" Thinking Mode", "", ActionToggleThinking{}). Siblings show
  theirs: toggle_sidebar gets "ctrl+b" (commands.go:488),
  file_picker gets "ctrl+f" (commands.go:495).
• ActionToggleThinking has exactly two references in the tree: the
  palette item and its handler (internal/ui/model/ui.go:2262). No key
  path reaches it.
• internal/ui/model/keys.go has no thinking/reasoning binding at all —
  quit, help, commands, models, sessions, yolo, sidebar, pills/tasks,
  details all have keys; thinking does not.

So every flip is ctrl+p, type "think", enter. Trading depth for speed
turn-by-turn (the workflow behind #2651) turns that into a lot of palette
trips.

Proposed change

Mirror the ToggleSidebar pattern (keys.go:221-224):

1. internal/ui/model/keys.go: add Chat.ToggleThinking key.Binding
   plus a DefaultKeyMap entry.
2. internal/ui/model/ui.go: key.Matches(msg, m.keyMap.Chat.ToggleThinking) next to the existing
   Chat.ToggleSidebar match (ui.go:3098), dispatching to the same
   code as the palette action, and add the binding to mainBinds
   (ui.go:4027) so it works while the editor is focused.
3. internal/ui/dialog/commands.go:475: pass the binding's key as the
   shortcut argument instead of "" so the palette shows it.

Safe to hit mid-run: the toggle is not busy-gated and applies on the next
turn via updateAgentModelCmd (ui.go:2262), same as the model and
effort pickers.

Scope: only meaningful where the palette item already appears (CanReason
with no ReasoningLevels, commands.go:470); a no-op elsewhere is fine.
Models with reasoning levels have the effort dialog instead — out of
scope here. The specific key is yours to pick; as of a48dabea plain
ctrl+h/ctrl+u/ctrl+w look unbound in the main view.

All line numbers are against upstream main @ a48dabea (2026-10-14).

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[andrinoff]

Moving this to a discussion, thank you!