[FEATURE] Auto model switching based on conversation context (plan vs. implementation)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Problem

When working on complex projects, the ideal workflow is:

• Planning/Conceptual work → cheaper/faster model (e.g. Sonnet)
• Implementation/Coding → most capable model (e.g. Opus)

Currently this requires manual /model <name> commands. It's easy to forget,
leading to either unnecessary cost (Opus for planning) or reduced code quality
(Sonnet for implementation).

Proposed Solution

Add a settings.json option for automatic context-aware model switching:

{
  "autoModelSwitch": {
    "planMode": "claude-sonnet-4-6",
    "implementationMode": "claude-opus-4-6"
  }
}


### Alternative Solutions

If automatic switching is too complex, a prompt when entering/exiting Plan
Mode would already be a huge improvement:

"You are entering Plan Mode. Switch to Sonnet to save costs? [y/N]"

### Priority

High - Significant impact on productivity

### Feature Category

Performance and speed

### Use Case Example

Example scenario:
1. I'm architecting a new feature for an enterprise AI platform (SPS TALOS)
2. I start planning: analyzing requirements, designing the approach, reviewing 
   existing code patterns — all conceptual work where Sonnet is sufficient
3. Once the plan is approved, I switch to implementation — now I want Opus 
   for maximum code quality, security awareness, and complex refactoring
4. Currently I have to manually type `/model opus` at exactly the right moment.
   I forget this regularly, burning Opus tokens on pure planning, or writing 
   complex security-critical code with Sonnet.

With this feature:
- Enter Plan Mode → automatically switches to Sonnet (saves ~5x cost)
- Approve plan / exit Plan Mode → automatically switches back to Opus
- Zero cognitive overhead, optimal cost/quality at every step

### Additional Context

_No response_