refactor(coding-agent): remove neo tui flag and binary

[code-yeongyu]

Summary

• Remove the --neo CLI flag and native neo TUI dispatch path.
• Remove neo TUI binary build/release/CI wiring and the Rust neo-tui workspace.
• Add a regression test that locks help output, rejected --neo, default CLI dispatch source, and package metadata.

Validation

• RED: npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/regressions/neo-flag-removal.test.ts failed 4 intended assertions before implementation.
• GREEN: same regression test passes 4/4 after implementation and after formatting.
• npm run check
• Surface artifacts: help omits --neo; --neo exits 1 with Error: Unknown option: --neo; --version still prints 2026.5.26; package metadata has no build:neo-tui.

---

Summary by cubic

Removed the experimental --neo CLI and native Rust TUI to simplify the coding-agent build and CI. The CLI now rejects --neo and only runs the default interactive mode; all Rust workspace code and wiring are removed.

• Refactors

  • Removed --neo parsing/dispatch from packages/coding-agent (help updated; --neo errors).
  • Deleted packages/neo-tui/, root Cargo.*, and all related assets/tests.
  • Dropped Rust setup and cargo steps from .github CI and publish workflows.
  • Simplified packages/coding-agent/package.json build script (removed build:neo-tui).
  • Added regression test locking help output, --neo rejection, default dispatch, and package metadata.

• Migration

  • Remove --neo from any scripts or aliases; use the default interactive TUI.
  • No Rust toolchain needed in CI/publish pipelines anymore.

^{Written for commit 27b17af. Summary will update on new commits. Review in cubic}