[chruffins]

Summary

• drain PTY output independently of WebSocket attachment and retain the latest 64 KiB in a fixed-size ring buffer
• forward output on a separate goroutine so WebSocket backpressure cannot stop PTY draining; flush pending output before the exit event
• add regression coverage for replay/exit ordering, ring-buffer reuse, and continued draining while the sink is blocked

Overlap with #463

Checked open PR #463 first. It changes process lifecycle and process.go, but its description explicitly excludes TTY sessions from output capture; it does not cover PTY attach replay or ordering. This PR changes the same file, so a merge conflict with #463 is possible. The PTY changes are otherwise separate from #463's non-TTY output lifecycle work.

Validation

• cd server && go test -race ./cmd/api/api -run 'TestPTYOutput' -count=10
• cd server && go test ./cmd/api/api -count=1
• cd server && go vet ./cmd/api/api

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