feat(cli): add /reasoning command for effort level and display toggle

Combined implementation of reasoning management:
- /reasoning              Show current effort level and display state
- /reasoning <level>      Set reasoning effort (none, low, medium, high, xhigh)
- /reasoning show|on      Show model thinking/reasoning in output
- /reasoning hide|off     Hide model thinking/reasoning from output

Effort level changes persist to config and force agent re-init.
Display toggle updates the agent callback dynamically without re-init.

When display is enabled:
- Intermediate reasoning shown as dim [thinking] lines during tool loops
- Final reasoning shown in a bordered box above the response
- Long reasoning collapsed (5 lines intermediate, 10 lines final)

Also adds:
- reasoning_callback parameter to AIAgent
- last_reasoning in run_conversation result dict
- show_reasoning config option (display section, default: false)
- Display section in /config output
- 34 tests covering both features

Combines functionality from PR #789 and PR #790.

Co-authored-by: Aum Desai <Aum08Desai@users.noreply.github.com>
Co-authored-by: 0xbyt4 <35742124+0xbyt4@users.noreply.github.com>

cli-config.yaml.example

@@ -670,6 +670,11 @@ display:
   # Works over SSH. Most terminals can be configured to flash the taskbar or play a sound.
   bell_on_complete: false
 
+  # Show model reasoning/thinking before each response.
+  # When enabled, a dim box shows the model's thought process above the response.
+  # Toggle at runtime with /reasoning show or /reasoning hide.
+  show_reasoning: false
+
   # ───────────────────────────────────────────────────────────────────────────
   # Skin / Theme
   # ───────────────────────────────────────────────────────────────────────────