feat(cli): add user-scoped host setup

[OpenAIBot1]

Summary

• add zenith init --scope user for deterministic cross-workspace setup in Claude Code and Codex
• install the user-scoped MCP registration, host agents/playbooks, orchestrator prompt, and personal /zenith skill while preserving unrelated user configuration
• keep project scope as the backward-compatible default and document user/project setup, both-host installation, restart behavior, config-directory overrides, and checkout relocation

User scope deliberately does not select or persist a model, reasoning effort, sandbox mode, feature preference, or ambient API/model environment variable. Existing project-scope behavior is unchanged.

This turns the global-skill direction introduced in #5 into a tested CLI workflow. Hermes remains available through project scope until its authoritative user-scope paths are established.

Closes #22.

Compatibility

• Claude Code: writes the root user MCP entry in ~/.claude.json or $CLAUDE_CONFIG_DIR/.claude.json
• Codex: writes only a managed Zenith MCP block in $CODEX_HOME/config.toml (default ~/.codex/config.toml)
• existing Claude JSON and Codex TOML preferences, unrelated MCP servers, skills, and agents are preserved
• existing managed or unmanaged Zenith registrations are replaced idempotently, including equivalent quoted/commented Codex TOML headers
• malformed or structurally incompatible host config fails before asset mutation
• Claude Code and Codex may be installed in either order

Validation

• uv run ruff check .
• uv run mypy src
• uv run pytest -q — 217 passed, 7 skipped
• fresh disposable-home CLI flows for Claude Code and Codex in both installation orders
• claude mcp get zenith reported user scope and connected
• codex mcp get zenith reported the server enabled
• registered absolute server command launched from an unrelated workspace
• omitted versus explicit project scope produced identical workspace trees

Checklist

• uv run ruff check . passes
• uv run mypy src passes
• uv run pytest -q passes
• Tests added/updated for behavior changes