feat(editors): add github-copilot-vscode adapter

[Jose-Sabater]

Summary

• Routes VS Code Copilot Chat through Opper via the johnny-zhao.oai-compatible-copilot community extension. First configure-only adapter in the registry — no spawn (the user opens VS Code themselves; Opper models appear in the Copilot Chat picker on next session).
• Surfaced as opper editors github-copilot-vscode and via the menu's agents flow.
• configure() prompts before installing the third-party extension, with a marketplace link and a clean cancel path. If the user declines, no settings.json is written.
• Settings merge is non-destructive: only oaicopilot.baseUrl and oaicopilot.models are written/overwritten; other oaicopilot.* user prefs are preserved; JSONC files (with // comments) throw rather than silently lose comments.
• Stable channel only — Insiders' native customOAIModels is upstream-deprecated behind a one-shot migration.
• docs/copilot-vscode/ keeps the manual recipe + a small generator that derives both Insiders and Stable JSON snippets from PICKER_MODELS for users without the CLI.

Out of scope (deliberate)

• API key automation. User pastes once via VS Code's Manage Models UI. The two automation paths (plaintext oaicopilot.apiKey.opper in settings.json vs writing to OS keychain) both have real trade-offs (Settings Sync exposure, undocumented keychain entry shape) — leaving for a follow-up PR once we've validated a safe path.
• Inline completions. BYOK only covers Chat and Agent mode; inline suggestions stay on GitHub's own service. Documented in command output and README.

Test plan

• npm run typecheck clean.
• npm test — 375/375 passing (12 new for this adapter, includes prompt-confirms / prompt-cancels paths).
• Sandboxed end-to-end: OPPER_EDITOR_HOME=/tmp/x npx tsx src/index.ts editors github-copilot-vscode writes the right block; --remove round-trips back to {}.
• Real VS Code: extension installs on confirm, settings populate, models surface in the Copilot Chat picker after manual API-key entry, traces land on Opper.
• Menu flow (opper → agents → GitHub Copilot (VS Code) → Configure) shows the install prompt and respects cancel.

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