feat(cli): add setup select for explicit multi-host plugin install

[knqiufan]

Which issue or RFC does this PR close?

Closes #1301

Rationale for this change

Multi-agent machines still have to run one fail-closed setup <host> command per integration. Concatenating those commands aborts on the first missing CLI. PATH-driven auto-install would also treat “the CLI is present” as “install this plugin,” including Claude prompt capture.

This PR adds an opt-in orchestration command that installs only the first-class hosts the user selects, keeps the existing single-target commands unchanged, and isolates one host failure from the others.

What changes are included in this PR?

• Add powercontext setup select over a shared first-class catalog: codex, claude-code, dsh, pi, hermes.
• Reuse the current installers. Do not scan PATH to build or filter the menu. Do not start the Server or launch a host process.
• Repeatable --host is the scriptable path. A TTY without --host prints the catalog and accepts numbers or names. --json and non-TTY require --host.
• Selected hosts that fail (missing CLI, unbuilt DSH plugin, Claude marketplace conflict) report failed and continue. Unselected hosts are skipped.
• Empty powercontext setup still prints help. setup <host> remains fail-closed. doctor is unchanged; doctor integrations is left for a follow-up PR.
• Docs: English and Chinese install, troubleshoot, and interfaces pages.
• Tests: tests/test_setup_select.py covers selection, isolation, TTY/non-TTY, JSON, flag pass-through, and the single-target fail-closed regression.

Are there any user-facing changes?

Yes. New optional command:

powercontext setup select --host codex --host dsh
powercontext setup select --host codex --json

No breaking API or storage-format change. Per-host setup commands keep their current behavior.

How was this change tested?

• python -m pytest tests/test_setup_select.py
• python -m pytest tests/test_system_cli.py tests/test_dsh_cli.py tests/test_hermes_cli.py tests/test_pi_cli.py
• ruff check on the changed Python files
• ty check on the changed Python files
• uv lock --locked

AI usage statement

Used for implementation and review against the issue contract. The design, command surface, and acceptance criteria follow #1301 and the follow-up comment on that issue.