feat(cli): check tmux prerequisite during setup

[codeaholicguy]

Motivation

Interactive managed agents require tmux, but users previously discovered that prerequisite only when agent start failed. This surfaces the dependency during setup while retaining an actionable runtime guard.

Design summary

• Add an injected, read-only inspectTmux check and pure platform instruction resolver.
• Render a separate Host Prerequisites block before agent setup.
• Reuse platform-aware guidance when interactive agent start encounters missing tmux.
• Cover inspection, distro mapping, setup output, and runtime messaging with mock-only tests.
• Document the prerequisite and provisional tmux 2.6+ floor; update the changelog and lifecycle docs.

Approved decisions

1. Missing tmux warns and setup continues.
2. No install flag or host mutation.
3. Versions are reported but never rejected.
4. Host prerequisites render separately from the per-agent table.
5. Only setup and the runtime guard change; init/install/doctor remain untouched.

Validation

• npm ci
• npm run build (all 6 projects; includes TypeScript declaration compilation)
• npm test (85 files, 1,042 tests)
• npm run lint (0 errors; 5 pre-existing unrelated warnings)
• npm run test:e2e (41 tests)
• npx ai-devkit@latest lint --feature tmux-setup-check
• targeted tmux pure-module coverage: 100% statements, branches, functions, and lines

Risks

No known host-mutation risk: the feature only executes tmux -V and prints fixed guidance. Unknown platforms fall back without guessing.