Make GPT-5.5 large prompt default

[jukasper]

Summary

• Make the GPT-5.5 large prompt the only/default GPT-5.5 prompt, matching the GPT-5.4 cleanup pattern.
• Remove the github.copilot.chat.gpt55LargePromptSections.enabled and github.copilot.chat.gpt55EconomicalSearchAndEdit.enabled experiment settings, config keys, and capability helpers.
• Delete the unused GPT-5.5 large-prompt and economical-prompt variant files so Gpt55PromptResolver always resolves to Gpt55Prompt for GPT-5.5 models.

Testing

• npm run precommit
• Lightweight TSX transpile check for the remaining GPT-5.5 prompt files

Note: cd extensions/copilot && npm run typecheck passed earlier on this branch, but rerunning the full Copilot typecheck after the final cleanup was blocked locally by disk-space limits while reinstalling Copilot dependencies.

[Copilot]

Pull request overview

This PR makes GPT-5.5’s “large prompt sections” the default by inlining those sections directly into the shared Gpt55PromptBase, and removes the now-obsolete experiment/config plumbing that previously toggled them. The only remaining GPT-5.5 prompt variant is the economical search-and-edit experiment, now layered on top of the default large-prompt baseline.

Changes:

• Removed the github.copilot.chat.gpt55LargePromptSections.enabled experiment setting and related capability helper (isGpt55LargePromptSectionsExp) + config key wiring.
• Deleted the unused GPT-5.5 large-prompt prompt variants (gpt55LargePrompt.tsx) and simplified the GPT-5.5 prompt resolver accordingly.
• Updated Gpt55PromptBase to always include the previously-gated “large prompt sections”, while keeping the economical search-and-edit variant behind its experiment.

Show a summary per file

File	Description
extensions/copilot/src/platform/endpoint/common/chatModelCapabilities.ts	Removes the now-unused GPT-5.5 large-prompt experiment capability helper.
extensions/copilot/src/platform/configuration/common/configurationService.ts	Removes the EnableGpt55LargePromptSections experiment-based config key.
extensions/copilot/src/extension/prompts/node/agent/openai/gpt55Prompt.tsx	Simplifies prompt selection to default base prompt vs. economical search/edit variant only.
extensions/copilot/src/extension/prompts/node/agent/openai/gpt55LargePrompt.tsx	Deletes obsolete large-prompt experiment prompt variants.
extensions/copilot/src/extension/prompts/node/agent/openai/gpt55BasePrompt.tsx	Makes the previously large-prompt-only sections part of the default GPT-5.5 base prompt.
extensions/copilot/package.nls.json	Removes the localized description string for the removed configuration key.
extensions/copilot/package.json	Removes the configuration contribution for the removed experiment setting.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 0