Make new plan mode default

[JonasHelming]

What it does

Simplifies the Architect agent by removing the default prompt mode and making the planning (plan-next) prompt the default. Removes associated unused code:

• Removed ARCHITECT_DEFAULT_PROMPT_ID and its prompt template
• Removed AI_SUMMARIZE_SESSION_AS_TASK_FOR_CODER and AI_UPDATE_TASK_CONTEXT_COMMAND commands
• Removed TaskContextAgent (no longer needed)
• Removed TaskContextService.update() method
• Removed task-context-prompt-template.ts (prompts no longer used)

How to test

1. Open a chat with @Architect
2. Verify the default mode is now "Plan Mode"
3. Verify "Simple Mode" is still available as an alternative
4. Create a plan using the architect - verify "Execute with Coder" suggestion appears
5. Confirm the old suggestions ("Summarize this session as a task for Coder", "Update current task context") no longer appear

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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