Support GitHub Copilot CLI as a first-class provider (via Copilot SDK / ACP)

[NK-Works]

Problem

GitHub Copilot subscribers can't use their subscription/quota in T3 Code. Prior requests (#193, #735, #282, #628, #288) are all closed, so there is currently no open tracking item. The workaround is OpenCode's github-copilot/* models, not a first-class Copilot provider.

Proposal

Add a first-class copilot provider driver that drives the Copilot CLI through the official Copilot SDK path (copilot --server JSON-RPC, managed by @github/copilot-sdk for TypeScript), with copilot --acp as the interim/fallback transport.

Suggested implementation (best way)

Recommended: new dedicated CopilotDriver, modeled on PiDriver (the simplest recent driver), not CodexDriver (which carries managed-home/auth-overlay/reset-credit complexity we don't need for v1):

1. packages/contracts/src/settings.ts: new CopilotSettings via makeProviderSettingsSchema — enabled off by default (like pi/cursor/grok), binaryPath default copilot, launchArgs, customModels.
2. apps/server/src/provider/Drivers/CopilotDriver.ts: implement ProviderDriver<CopilotSettings, CopilotDriverEnv>, register in builtInDrivers.ts. Keep per-instance state in create() scope, no shared mutable state across instances.
3. apps/server/src/provider/Layers/CopilotProvider.ts + CopilotInstallation.ts: snapshot/probe (copilot --version, auth status, models.list), following Layers/PiProvider.ts + providerMaintenance.ts with makePackageManagedProviderMaintenanceResolver({ npmPackageName: "@github/copilot" }).
4. apps/server/src/orchestration-v2/Adapters/CopilotAdapterV2.ts: session/turn/approval runtime. Prefer the official @github/copilot-sdk TS client (it owns CLI process lifecycle, protocol v2–v3 negotiation, models.list, BYOK) over reimplementing JSON-RPC. Model the adapter shape on PiAdapterV2.
5. textGeneration/CopilotTextGeneration.ts: commit/PR/branch/title generation via the same runtime, like PiTextGeneration / CodexTextGeneration.
6. Auth: terminal login (copilot login) as an ACP-style terminal auth method plus env-var BYOK path, reusing the existing ProviderAuthController pattern.
7. Docs + contracts only where they cross the wire; no changes to apps/web/src/components/ui styling.

Interim workaround worth documenting: copilot --acp already works through the generic AcpRegistryDriver. Note CLI 1.0.88 fixed managed settings for --acp/--server, so a registry entry is a reasonable stopgap until the native driver lands.

Phasing I'd propose: (1) settings + snapshot/installation probe + disabled-by-default flag, (2) adapter turns/approvals via SDK server mode, (3) text generation + maintenance polish. Focused tests per behavior (vp test run <files>), no repo-wide checks.

Prior art / compatibility notes

• SDK communicates with the CLI over JSON-RPC server mode; CLI-only interactive features need explicit protocol exposure to be SDK-reachable (upstream github/copilot-sdk docs: SDK and CLI compatibility).
• Upstream github/copilot-sdk currently has open CLI/SDK interop reports worth watching (e.g. session.idle semantics, trace-span parenting across SDK/CLI, maxContextWindowTokens handling, shutdown destroy rejection on SDK 1.0.14/runtime 1.0.85).

May I work on this? Happy to start with the settings + snapshot/probe spike behind the disabled-by-default flag and bring back a scoped proposal/PR per CONTRIBUTING.md.

[NK-Works]

@juliusmarminge tagging you for your agent's visibility :)

This one is personal because I get monthly 50K+ credits on copilot but don't want to use that garbage cli thought maybe T3 code will finally allow me to consume those sitting tokens as well 😅