feat(terminal): add Ptyxis support - #113

Summary

• add a Linux ptyxis terminal preset that opens agents in a new Ptyxis window
• auto-detect Ptyxis sessions through PTYXIS_VERSION
• strip and capture Ptyxis identity variables consistently across launches and relay tests
• expose Ptyxis in Linux fallback selection, the TUI preset list, config validation, and documentation

Ptyxis is intentionally open-only: its CLI can open windows and tabs, but does not expose a supported pane/window ID close API for managed hcom kill cleanup.

Testing

• cargo fmt --all -- --check
• bash ./scripts/typecheck.sh
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked (2175 unit tests passed; integration suites passed)
• cargo +1.88 check --locked --all-targets (MSRV)
• pinned Codex real-tool tests (2 passed)
• pinned Claude real-tool tests (2 passed)
• MQTT relay roundtrip test (all 14 phases passed)
• manual smoke test with ptyxis --new-window -- bash -c 'sleep 3'