Add ghost init command for interactive post-install setup

[murrayju]

Summary

• Adds ghost init, an interactive multi-select that walks the user through adding Ghost to $PATH, logging in, installing the MCP server for one or more clients, and installing shell completions. Each step shows its detected status (e.g. already in PATH (~/.local/bin), already configured (you@example.com)) and is pre-selected only if not already configured.
• Adds ghost init path — a non-interactive subcommand that appends the install dir to the user's shell rc file. Suitable for scripts.
• Adds --skip-if-configured so ghost init is safe to re-run from the installer; it exits cleanly when nothing needs to change.
• scripts/install.sh now delegates all post-install setup (PATH, login, MCP, completions) to ghost init, removing ~200 lines of shell-side rc-file logic that previously duplicated what the Go code can now do correctly. The installer falls back to ghost init path when /dev/tty isn't usable.
• ghost mcp install and ghost mcp uninstall now support multi-client selection via the same shared multi-select prompt — pick any combination interactively, or pass all / a specific client name as before.
• Extracts the multi-select UI into internal/common/multiselect.go and the shell rc / PATH / completion helpers into internal/common/shell_setup.go, both with unit tests.
• Adds a ./check script that runs go install, fmt, mod tidy, fix, vet, staticcheck, and test in one shot; CLAUDE.md updated to reference it.
• Regenerated CLI reference docs under docs/cli/.

Test plan

• ./check passes
• Fresh install via curl -fsSL https://install.ghost.build | sh walks through the new menu on a machine with no prior Ghost config
• Re-running the installer with everything already configured exits quickly (the --skip-if-configured path)
• ghost init with stdin redirected returns the TTY error instead of hanging
• ghost init path works under sh -c with no TTY (smoke test for the script-friendly fallback)
• ghost mcp install with no args shows the multi-select; selecting multiple clients installs into all of them
• ghost mcp uninstall with no args shows the multi-select; selected clients are removed and others are left alone
• Esc in the MCP submenu of ghost init exits the flow cleanly with "Canceled."
• Ctrl+C anywhere in ghost init exits without a stack trace

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