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

---

Note

High Risk
Touches checkpoint revert ordering, provider rollback recovery, and a new full provider adapter—errors could leave filesystem and conversation state inconsistent or mis-project active turns.

Overview
Adds GitHub Copilot as a built-in provider via CopilotDriver, the official SDK dependencies, and a large CopilotAdapter surface (sessions, permissions, event normalization, diffs, titles, background tasks) with extensive adapter tests.

Checkpoint and revert behavior is tightened: missing placeholder git refs are only captured once the session is no longer running, with a thread.session-set path to flush remaining placeholders; revert restores the filesystem before provider rollbackConversation, rolls back the workspace on provider failure (with optional restore to the current checkpoint), skips provider rollback when filesystem restore fails, and preflights checkpoint ref availability. Checkpoint diffs return a clearer error when a ref is missing.

Orchestration/projections avoid advancing latestTurn while another turn is still active, map content.delta streams into reasoning and tool-output activities, and improve first-turn title/branch generation (drain waits on title jobs, retries, truncated-seed matching, Copilot model selection in tests).

Test harnesses move some vi imports to vitest and add regression coverage across checkpoint store, checkpoint reactor, provider command reactor, and runtime ingestion.

^{Reviewed by Cursor Bugbot for commit df43adf. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add GitHub Copilot as a built-in provider with adapter, text generation, and settings

• Introduces CopilotDriver as a registered built-in provider, wiring together a CopilotAdapter for session/turn/approval flows and makeCopilotTextGeneration for commit messages, PR content, branch names, and thread titles
• Adds CopilotSettings to server config (binary path, server URL, custom models) and exposes Copilot in the settings UI; removes GitHub Copilot from the "coming soon" list
• createCopilotClient handles CLI path resolution, POSIX shell normalization, environment setup, and model capability mapping (reasoning effort, context tier)
• Text generation reuses a shared client per (cwd, settings) key, creates isolated sessions per request, retries on failure, and shuts down idle clients after 30 seconds
• Checkpoint revert now validates target ref availability, attempts provider rollback on failure, restores the filesystem to the current checkpoint, invalidates workspace entries, and records detailed failure activities
• Risk: adds @github/copilot and @github/copilot-sdk as runtime server dependencies; Copilot sessions time out at 180s with no streaming support

^{Macroscope summarized df43adf.}

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

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[macroscopeapp]

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[macroscopeapp]

🟡 Medium Layers/CopilotAdapter.ts:1855

event.data.message.trim() throws when event.data.message is null or undefined. Other handlers in this file use trimOrUndefined() to safely handle optional SDK messages.

-          payload: {
-            message: event.data.message.trim(),
-            detail: event.data,
-          },
+          payload: {
+            message: trimOrUndefined(event.data.message) ?? "",
+            detail: event.data,
+          },

🤖 Copy this AI Prompt to have your agent fix this:

In file @apps/server/src/provider/Layers/CopilotAdapter.ts around lines 1855-1858:

`event.data.message.trim()` throws when `event.data.message` is `null` or `undefined`. Other handlers in this file use `trimOrUndefined()` to safely handle optional SDK messages.

Evidence trail:
apps/server/src/provider/Layers/CopilotAdapter.ts line 1855 (direct .trim() on event.data.message), line 1761 (same field uses trimOrUndefined for session.error), apps/server/src/provider/copilotRuntime.ts lines 63-65 (trimOrUndefined definition with null-safe ?.trim())