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

High Risk
Large new Copilot adapter surface plus changed checkpoint revert ordering affects filesystem and provider state together; mistakes could leave workspaces inconsistent after failed reverts.

Overview
GitHub Copilot is wired in as a server provider via @github/copilot / @github/copilot-sdk, with a new CopilotDriver and a large CopilotAdapter test suite covering sessions, permissions, diffs, task output, queued turns, and lifecycle edge cases.

Checkpoint revert now verifies the target git ref exists, restores the filesystem checkpoint before calling rollbackConversation, and on provider rollback failure attempts to restore the pre-revert checkpoint, records a failure activity, and invalidates workspace entries; provider rollback is skipped when the initial filesystem restore fails.

Read-model / UX fixes: latestTurnId and in-memory latestTurn no longer advance when a different turn is still running; turn-diff completion respects the same rule. First-turn helpers enqueue title and branch generation on a drainable worker (title generation retries, replaces titles that match a truncated client seed), and reactor drain waits for that work. Runtime ingestion maps content.delta streams into reasoning.* and tool.output thread activities. Checkpoint diff errors for missing refs are clearer in tests.

^{Reviewed by Cursor Bugbot for commit b589b84. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

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

• Adds a new copilot driver kind end-to-end: settings schema, provider registry entry, UI provider picker, model picker sidebar, and icon mapping
• Implements CopilotAdapter to integrate the GitHub Copilot SDK: starts/stops a client, creates/resumes sessions, translates SDK events into ProviderRuntime events, and manages turn lifecycle including diffs and token snapshots
• Implements CopilotTextGeneration for commit messages, PR content, branch names, and thread titles using JSON-schema-constrained Copilot sessions with client reuse and idle auto-shutdown
• Resolves the Copilot CLI binary via serverUrl > configured binaryPath > bundled shim under node_modules/.bin/copilot, normalizing POSIX SHELL and COPILOT_FEATURE_FLAGS at runtime
• Fixes latestTurn/latestTurnId settlement in the projector and store so in-progress turns are not prematurely overwritten by concurrent checkpoint or diff events
• Risk: adding @github/copilot and @github/copilot-sdk as server dependencies increases the bundle; the bundled CLI shim path resolution searches up to 8 ancestor directories of the module root at startup

^{Macroscope summarized b589b84.}

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