refactor: extract shared shell argument builder to eliminate duplication

[jongio]

Code Smell: Duplicated Code (Dispensables)

Severity: HIGH
Category: Dispensables > Duplicated Code
Files: cli/src/cmd/exec/commands/mcp.go, cli/src/internal/executor/command_builder.go

Description

The shell argument construction logic is duplicated between two independent implementations:

1. buildShellArgs() in mcp.go (lines 381-410) — used by MCP handlers
2. buildCommand() in command_builder.go (lines 31-79) — used by the CLI executor

Both functions contain the same switch-case structure handling bash/sh/zsh, pwsh/powershell, and cmd shells with identical argument patterns. When a new shell is added or argument behavior changes, both locations must be updated independently, risking divergence.

Current divergence example: The MCP version adds -NoProfile for PowerShell but the CLI version does not. This may be intentional, but having two independent implementations makes it unclear whether differences are bugs or features.

Refactoring Recommendation

Extract a shared ShellCommandBuilder in a common package (e.g., internal/shellcmd) that both the CLI executor and MCP handlers can use:

// Package shellcmd builds shell-specific command argument lists.
package shellcmd

type Options struct {
    NoProfile bool // Add -NoProfile for PowerShell (used by MCP)
}

func BuildArgs(shell, scriptOrCmd string, isInline bool, extraArgs []string, opts Options) []string {
    // Single implementation of shell argument construction
}

This eliminates the duplication and makes shell behavior differences explicit through options rather than implicit through separate implementations.

Impact

• Maintenance burden: Changes to shell handling require updating two files
• Bug risk: Divergent behavior between CLI and MCP code paths is hard to detect
• Testing overhead: Both implementations need independent test coverage for the same logic