Restore ANSI colors for agent-launched terminals

[dudetru25]

What changed

• sanitize the PTY environment when Unity is launched from a non-interactive TERM=dumb host
• clear inherited NO_COLOR only for that host case, while preserving an explicit preference from interactive terminals
• keep advertising xterm-256color and truecolor support
• add regressions for PTY environment behavior and ANSI red/reset parsing

Root cause

Unity inherited TERM=dumb and NO_COLOR=1 from the agent host. Unterm replaced TERM and COLORTERM, but portable-pty also inherited NO_COLOR, so Codex correctly suppressed SGR color sequences before they ever reached Unterm's parser and renderer.

Impact

Codex and other color-aware terminal applications can render semantic colors, including red interruption/error text, when Unity was started from an agent or build environment. Users who intentionally set NO_COLOR from a normal interactive terminal keep that preference.

Validation

• cargo test -p unterm --lib --locked — 58 passed, 1 ignored
• repository CI-equivalent cargo clippy -p unterm --lib --bins --tests --locked with the workflow's documented legacy-lint allowances — passed
• focused rustfmt --edition 2021 --check unterm/src/pty.rs — passed
• independent PTY reproduction: Codex emitted 0 escape bytes with NO_COLOR=1 and 128 after NO_COLOR was unset

The repository-wide formatter and all-target example build still expose pre-existing unrelated debt (dump_selection references the removed unterm_get_pixels API); this PR does not change those paths.

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