[ax] Extract TerminalDetector helper for TTY detection

[TomasVotruba]

Why

TTY detection was inlined in three places, each hand-rolling stream_isatty(...):

• ConfigurationFactory (progress bar off on non-TTY, [ax] Disable progress bar when output is not a TTY #8444)
• SymfonyStyleFactory (no ANSI on non-TTY, [ax] Never emit ANSI when output is not a TTY, even with --ansi #8453)
• ConfigInitializer (no config prompt on non-TTY, [ax] Do not prompt or auto-create config on non-TTY #8454)

As Rector grows more agent-aware, this check needs one home.

What

New Rector\Agentic\TerminalDetector with two static checks:

TerminalDetector::isOutputTty(); // STDOUT - progress bar, ANSI
TerminalDetector::isInputTty();  // STDIN  - interactive prompts

The three call sites now use it. Behavior is unchanged - static, matching the existing StaticPHPUnitEnvironment utility pattern, since SymfonyStyleFactory is built manually at bootstrap (no container).

New src/Agentic/ namespace is the home for agent-oriented helpers going forward.

Verification

• Piped --ansi run still emits 0 ANSI escapes; non-TTY still skips the config prompt.
• composer check-cs, composer phpstan clean.