fix(extended-thinking): correct fictional /think command to valid ultrathink keyword (#83)

- Replace fictional `/think` example with official `ultrathink` keyword
- Fix Q4 options: `/think` → `/effort max`, `--thinking` → `ultrathink`
- Fix Q5: correct the answer that "ultrathink" DOES trigger deep reasoning
- Sync changes across English, Ukrainian, and Vietnamese translations

Background: `/think` is not a valid slash command in Claude Code (verified
against official commands reference at code.claude.com/docs/en/commands).
The correct way to trigger deep reasoning for a single response is to
include "ultrathink" in the prompt.

Sources:
- https://code.claude.com/docs/en/commands (no /think command exists)
- https://code.claude.com/docs/en/model-config (ultrathink is valid keyword)

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

vi/09-advanced-features/README.md

@@ -257,7 +257,7 @@ claude --effort high "complex architectural review"
 ### Example: Architectural Decision
 
 ```
-User: /think Should we use microservices or a monolith for our e-commerce platform?
+User: ultrathink Should we use microservices or a monolith for our e-commerce platform?
 
 Claude: Let me think through this architectural decision carefully...