includeCoAuthoredBy: false is not inherited by task subagents; their commits still get the Co-authored-by trailer

Describe the bug

includeCoAuthoredBy: false in ~/.copilot/settings.json is respected by the interactive session, but is not inherited by task-tool subagents. Commits created by subagents still get the Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> trailer.

This is a follow-up to #3181, which added the opt-out. The setting works for the main session — it just doesn't reach spawned subagents.

Beyond silently ignoring the setting, subagents appear to be told the trailer is mandatory. During a plan-execution run, one subagent amended an otherwise-finished commit purely to add it, and logged:

note: amended cleanup commit to <sha> to add the required Co-authored-by trailer; no file changes.

That "required" wording suggests the subagent's system prompt carries the coauthor instruction unconditionally, rather than gated on coauthorEnabled.

Affected version

GitHub Copilot CLI 1.0.80

Steps to reproduce the behavior

1. Set the opt-out in ~/.copilot/settings.json:

   { "includeCoAuthoredBy": false }
   

2. Start an interactive session in a git repo.
3. Ask the main session to make a commit directly (e.g. via a shell git commit). ✅ No trailer — setting respected.
4. Now delegate committing work to a subagent, e.g. via the task tool with agent_type: "general-purpose", instructing it to implement a change, commit it, and stop.
5. Inspect the resulting commit:

   git log -1 --format='%b'
   

6. ❌ The message contains Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>.

In my case 8/8 commits created by subagents carried the trailer, while every commit made directly from the main session had none. Stripping them across the branch required:

git rebase <base> --exec "git log -1 --pretty=%B | grep -v '^Co-authored-by:' | git commit --amend --quiet -F -"

Expected behavior

includeCoAuthoredBy: false should apply to all commits produced by the CLI, including those created by subagents — the setting reads as global, not session-scoped.

Additional context

• The setting key itself is correct and current: app.js computes coauthorEnabled: T.includeCoAuthoredBy !== false, and includeCoAuthoredBy?: boolean is declared in sdk/index.d.ts. (The changelog entry for #3181 calls it include_coauthor, which no longer matches the settings.json key — minor doc drift, possibly worth fixing too.)
• Ruled out as causes in my repo: no git hooks, core.hooksPath unset, and commit.template pointed at an empty file. Raw git commit from the main session never adds the trailer.
• settings.json had includeCoAuthoredBy: false for ~2 weeks before the affected session, so this isn't a case of the setting being applied late.
• Environment: macOS 26.5.2, arm64, zsh, iTerm2/agterm.

coauthorEnabled が計算される app.js と、includeCoAuthoredBy が宣言される sdk/index.d.ts から始めます。task-tool のサブエージェントが設定と coauthor の指示をどのように受け取るかを追跡し、次に includeCoAuthoredBy を false に設定して再現し、git log を調べます。無効時にサブエージェントが作成したコミットで trailer が省略され、デフォルトの動作は有効なままであれば完了です。