[codex] Allow neo TUI sentinel flag forwarding

[IYENTeam]

What changed

• Fixed parseArgs so senpi --neo -- ... stops Node-side parsing at the sentinel and lets neo-mode forward the tail to the Rust binary.
• Added a regression test covering --neo -- --list-themes --theme dracula parser behavior.
• Updated the existing neo forwarding regression comment and changelog entry.

Why

splitNeoArgs already documented and tested the backend-vs-Rust split, but the main CLI parser rejected Rust-only flags before runNeoMode could execute. This made documented commands like senpi --neo -- --list-themes fail at runtime.

Validation

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/regressions/neo-tui-arg-parse.test.ts test/suite/regressions/neo-tui-arg-forwarding.test.ts
• npm run check
• npm run build
• tmux runtime smoke: SENPI_NEO_TUI_DEV=1 node packages/coding-agent/dist/cli.js --neo -- --list-themes printed bundled theme ids and exited 0.

Not tested

• Full interactive agent session through the native TUI.

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Summary by cubic

Fixes senpi --neo -- ... so Rust TUI flags after the sentinel are passed through to the native binary instead of being rejected by the Node CLI parser. This restores the documented neo TUI contract and unblocks commands like senpi --neo -- --list-themes.

• Bug Fixes
  • Stop parsing at -- when --neo is set; forward the tail to neo-mode for Rust TUI handling.
  • Preserve existing sentinel behavior when not in neo mode (tail collected as messages).
  • Added regression tests for --neo -- --list-themes --theme dracula and updated changelog.

^{Written for commit 7b2ac6c. Summary will update on new commits. Review in cubic}