fix: guard _read_line_nonblocking against sys.stdin is None (#1170)

[microsasa]

Closes #1170

Problem

When sys.stdin is None (e.g. process started with closed stdin, or certain test/CI environments), select.select([None], [], [], timeout) raises a TypeError that isn't caught by the except (ValueError, OSError) handler in _interactive_loop. This crashes the interactive session instead of falling back to the threaded reader.

Fix

Added a sys.stdin is None guard at the top of _read_line_nonblocking that raises ValueError("stdin is None"), which the existing except (ValueError, OSError) handler in _interactive_loop already catches and handles via the threaded fallback.

Changes

1. src/copilot_usage/cli.py — Added if sys.stdin is None: raise ValueError("stdin is None") guard and updated docstring to document sys.stdin is None as a trigger condition
2. src/copilot_usage/docs/implementation.md — Updated fallback documentation to include sys.stdin is None as a third triggering case
3. tests/copilot_usage/test_cli.py — Added:
   • test_none_stdin_raises_value_error in TestReadLineNonblocking class (unit test)
   • test_interactive_loop_none_stdin_falls_back_to_input in Gap 3 section (integration test)

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[Copilot]

Pull request overview

This PR hardens the interactive CLI input path so _interactive_loop() no longer crashes when sys.stdin is None, which can happen in closed-stdin or CI/test environments. It fits the CLI’s existing fallback design by converting that condition into the same exception path already used for non-selectable stdin.

Changes:

• Added a guard in _read_line_nonblocking() to raise ValueError when sys.stdin is None.
• Added unit and interactive-loop tests covering the new stdin-none path.
• Updated implementation docs to mention sys.stdin is None as a fallback trigger.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/copilot_usage/cli.py	Adds the stdin-none guard in the nonblocking reader docstring and implementation.
src/copilot_usage/docs/implementation.md	Updates the interactive-loop design docs for the new fallback trigger.
tests/copilot_usage/test_cli.py	Adds regression tests for _read_line_nonblocking() and interactive fallback behavior.