Direct keybindings for reasoning effort level (e.g. Option+1/2/3)

Problem

Changing reasoning effort currently requires opening the model picker (Cmd+P) and then using arrow keys. For users who switch effort levels frequently, this is too many steps.

Proposed solution

Add keybindable actions that set effort level directly:

• chat:setEffortLow
• chat:setEffortMedium
• chat:setEffortHigh

These would work from the main chat context (no need to open the model picker first). Users could then bind them in ~/.claude/keybindings.json, e.g.:

[
  { "key": "alt+1", "command": "chat:setEffortLow" },
  { "key": "alt+2", "command": "chat:setEffortMedium" },
  { "key": "alt+3", "command": "chat:setEffortHigh" }
]

Context

• Current modelPicker:decreaseEffort / increaseEffort only work inside the picker dialog
• Cmd+T toggles thinking on/off but doesn't control effort level
• Related: #24187 (effort via /model command), #7668 (thinking mode config)