fix: frame intentional machine detach outcomes

[schickling-assistant]

Why

Machine attach consumers currently see EOF without a terminal protocol outcome when a user intentionally detaches with Ctrl+\. That is indistinguishable from truncation, even though the attach command exits successfully.

Closes #151.

What

Emit the existing empty DETACH frame as the terminal machine-stream outcome for intentional local detach. Preserve EXIT for a session-process exit and preserve EOF-without-outcome for transport truncation.

How

A single guarded detach finalizer first queues DETACH on the machine descriptor, then notifies and disconnects the session socket, ends the machine writer without waiting on backpressure, and invokes the existing detach callback after the writer flushes. It accepts DETACH before the initial GEOMETRY/SCREEN baseline and prevents a pending detach timer from producing a second outcome after EXIT.

Proof

• Public reproduction: schickling-repros/2026-07-pty-machine-detach-outcome
• Rebased onto merged test: make broken attach stream fixture protocol-valid #145 at upstream main 1c625c7
• Shipped-launcher detach regression, detach before the initial baseline, and EXIT winning the pending detach window
• npm run build: passed
• npm run typecheck: passed
• Attach-stream suite: 17/17 passed
• git diff --check origin/main...HEAD: passed
• Hosted Nix and Test: required on exact head b58db0c

Posted on behalf of @schickling

field	value
agent_name	co1-sage
agent_session_id	ed878dac-3735-4276-b3e0-ea1b1cd65291
agent_tool	Codex CLI
agent_tool_version	0.145.0
agent_runtime	Codex CLI 0.145.0
agent_model	unknown
runtime_profile	/nix/store/mnx8agbdq3wiyb6vz63lhgscgazkrn98-coding-agent-runtime-profile/share/coding-agents/profile.json
skills_manifest	/nix/store/5r69m9k2llmri3na81518zx0a7y0d3cn-agent-skills-corpus/share/agent-skills/manifest.json
worktree	pty/schickling-assistant/2026-07-31-machine-detach-outcome
machine	dev3
tooling_profile	dotfiles@0fb7e03

[schickling-assistant]

Merged with the repro, protocol contract, and exact outcome tests all aligned. Thank you, Nathan, for the careful sequencing and review — intentional detach is finally distinguishable from transport loss.

Posted on behalf of @schickling

field	value
agent_name	co1-sage
agent_session_id	ed878dac-3735-4276-b3e0-ea1b1cd65291
agent_tool	Codex CLI
agent_tool_version	0.145.0
agent_runtime	Codex CLI 0.145.0
agent_model	unknown
runtime_profile	/nix/store/mnx8agbdq3wiyb6vz63lhgscgazkrn98-coding-agent-runtime-profile/share/coding-agents/profile.json
skills_manifest	/nix/store/5r69m9k2llmri3na81518zx0a7y0d3cn-agent-skills-corpus/share/agent-skills/manifest.json
worktree	dotfiles/schickling/2026-07-20-cos-misc
machine	dev3
tooling_profile	dotfiles@0fb7e03