Add GitHub Copilot as a built-in provider

[huxcrux]

This PR supersedes #2185. It includes the full Copilot integration from that PR plus the fixes and hardening discovered while testing it against the current codebase.

What Changed

• Added GitHub Copilot as a first-class provider using the official Copilot SDK.
• Wired Copilot through the server runtime for auth/status checks, model discovery, session start/resume, turn sending, permission replies, request handling, and event streaming.
• Added Copilot-backed git text generation for commit messages, PR descriptions, branch names, and thread titles.
• Added Copilot provider settings, defaults, model traits, reasoning effort, context window handling, and persisted configuration.
• Updated the web UI so Copilot appears in provider/model pickers, settings, icons, context window selection, task rendering, and git writing model selection.
• Hardened Copilot runtime behavior for queued turns, duplicate completions, tool-only turns, empty diffs, missing checkpoint refs, stale resume cursors, failed sends, task completions, and rollback-safe checkpointing.
• Added regression coverage for the Copilot adapter, provider probing, text generation, context window handling, and related projection flows.

Why

T3 Code supports multiple agent providers, but did not have GitHub Copilot support. This adds Copilot end to end in the same shape as the existing providers, so users can configure Copilot, select Copilot models, use Copilot for git text generation, and rely on the same session/runtime flows as other providers.

The extra hardening keeps Copilot predictable under real usage: turns complete once, tool output and reasoning project correctly, diffs are only emitted when meaningful, permission replies resolve cleanly, checkpoint handling is safer, and provider status/model data stays accurate.

UI Changes

• Added Copilot to the provider/model picker.
• Added Copilot settings and model configuration surfaces.
• Added Copilot icons, reasoning/context-window controls, and task output rendering.

Updated model picker:

Provider settings:

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Medium Risk
Touches checkpoint revert ordering and provider conversation rollback (filesystem + provider state), plus turn-completion matching logic that affects session readiness across providers.

Overview
Adds GitHub Copilot as a server provider via a new CopilotDriver and @github/copilot / @github/copilot-sdk dependencies, wiring snapshot, adapter, and text generation like other built-in drivers.

Checkpoint revert now verifies the target ref exists, restores the filesystem before calling provider rollback, and skips rollback when filesystem restore fails. If rollback fails after a successful restore, it attempts to restore the current checkpoint, refreshes workspace entries, and records a failure activity instead of leaving a half-applied revert.

Runtime ingestion accepts turn.completed events without a turnId when the active turn still matches the provider session (or the provider has already cleared the active turn), and maps content.delta streams into reasoning and tool output activities.

First-turn orchestration queues title/branch generation on a drainable worker (with retries), broadens when auto-generated titles may replace truncated or provider-expanded seeds, and keeps latestTurnId on thread.session-set when activeTurnId is null.

^{Reviewed by Cursor Bugbot for commit 790af4a. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add GitHub Copilot as a built-in provider with full text generation and adapter support

• Registers a new copilot provider driver (CopilotDriver.ts) and wires it into the built-in driver registry, making GitHub Copilot selectable in the UI alongside other providers.
• Adds CopilotTextGeneration.ts for commit messages, PR content, branch names, and thread titles (titles are generated locally without starting a Copilot SDK session).
• Implements CopilotAdapter.ts and CopilotProvider.ts for session lifecycle, status probing, auth snapshots, and periodic refresh.
• Adds CopilotToolClassification.ts to map Copilot tool events to typed lifecycle categories (file change, command execution, web search, etc.).
• Extends contracts (settings.ts, model.ts) with Copilot-specific settings schema, default models (gpt-4.1), and display name mappings.
• Fixes deriveLatestContextWindowSnapshot to prefer the latest bounded window snapshot, stop at compaction events, and fall back to token-only snapshots when no limit is available.
• Fixes non-effort select descriptors (e.g. contextTier, variant) from incorrectly setting promptEffort in the composer state.

^{Macroscope summarized 790af4a.}

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[macroscopeapp]

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Verdict: Needs human review

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[macroscopeapp]

Effect service conventions review for the new GitHub Copilot provider. The previously-flagged import-namespace, CopilotCliPathResolutionError, and TextGenerationShape items now look resolved at this head. Two error-convention issues remain in changed scope.

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

Effect service conventions: one new finding in CopilotTextGeneration.ts. Previously flagged issues (CopilotProbePromiseError in copilotRuntime.ts and the inline error-construction helpers in CopilotAdapter.ts) remain unaddressed but are not re-commented here to avoid duplication.

^{Posted via Macroscope — Effect Service Conventions}