feat: migrate terminal to TypeScript 7

[AetherAI3]

Summary

• Migrate the full terminal codebase to TypeScript 7.0.2 and its native compiler workflow.
• Adopt Node 24 as the supported runtime baseline across package metadata, CI, install scripts, and contributor documentation.
• Simplify TypeScript source where the new compiler makes legacy assertions and compatibility code unnecessary.
• Add a migration guard covering the supported toolchain and runtime contract.
• Remove obsolete internal planning, audit, and draft-report files that should not ship in the public repository.
• Refresh the upgrade design with the implemented architecture and verification evidence.

Impact

This keeps terminal behavior stable while reducing build friction, removing the obsolete tsx development runtime, and making the repository's supported runtime/toolchain explicit.

Validation

• TypeScript 7.0.2 type-check passes
• Full test suite passes: 674 tests
• Node 24 test run passes
• Development and CLI smoke checks pass
• Package dry-run validation passes
• Dependency audit reports 0 vulnerabilities
• GitHub Actions passes on this commit

Spec: docs/specs/2026-07-20-typescript-7-terminal-upgrade-design.md