Fix interactive key reader fd flags

[dapi]

Fixes #33.

What changed

The Unix interactive key reader now opens the controlling terminal as a separate file description before enabling O_NONBLOCK. Teardown restores and closes only that reader descriptor, leaving stdin and any stdout/stderr descriptors sharing the original open-file description untouched.

Root cause

O_NONBLOCK is an open-file-description flag. Setting it on stdin made stdout non-blocking when both descriptors shared that description, allowing normal terminal backpressure to abort the workflow.

Routing and validation

• Flow: Bug Fix — the issue documents an existing expected behavior and a reproducible regression.
• Validation profile: standard — executable terminal-I/O behavior; no security, persistent-data, deployment, or cross-system triggers.
• Regression coverage: models a stdout descriptor sharing stdin open-file-description flags and proves key-reader setup does not change them.

Verification

• go test ./internal/terminal
• go test ./...
• go vet ./...
• make docs-lint
• git diff --check

No manual terminal smoke was run in the non-interactive execution environment.