Use stream_isatty for CLI stdin detection

[dereuromark]

Problem

bin/djot uses posix_isatty(STDIN) to detect whether stdin is a terminal (for the interactive hint and the legacy bare-stdin convert path). ext-posix is not a hard dependency of the package, so on PHP builds without it - Windows, or minimal/container PHP installs - the CLI fatals:

Fatal error: Call to undefined function posix_isatty()

instead of converting the input.

Fix

Replace both posix_isatty(STDIN) calls with the built-in stream_isatty(STDIN), which is part of PHP core since 7.2 and requires no extension. Behavior is identical where posix was available, and now works everywhere.

Added two CLI tests that run bin/djot with posix_isatty disabled (-d disable_functions=posix_isatty), covering both the convert - subcommand and the legacy bare-stdin form.

Context

Surfaced while updating the djot-intellij plugin to call this CLI: the plugin needs a fallback specifically because of this crash. With this fix the CLI is safe to invoke on any supported PHP build.

Note

phpstan reports one pre-existing error in Parser/BlockParser.php:589 (offsetAccess.invalidOffset) on master, unrelated to this change - left untouched.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (ae55a79) to head (fc25f4a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #240   +/-   ##
=========================================
  Coverage     91.89%   91.89%           
  Complexity     3527     3527           
=========================================
  Files           106      106           
  Lines          9970     9970           
=========================================
  Hits           9162     9162           
  Misses          808      808           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.