Harden CLI external-tool execution paths against command injection

[Copilot]

Sighthound identified critical command-injection risks in several pkg/cli paths where external tool commands were assembled from dynamic input. This change hardens those call sites by eliminating shell-style command construction and enforcing explicit argv-based execution.

• Security hardening in CLI exec call sites

  • Reworked vulnerable paths in:
    • pkg/cli/runner_guard.go
    • pkg/cli/grant.go
    • pkg/cli/poutine.go
    • pkg/cli/upgrade_command.go
  • Removed shell-interpolated command building patterns and switched to direct executable + argument-slice invocation.

• Argument handling constraints

  • Preserved existing behavior while tightening construction of command arguments.
  • Ensured dynamic values are passed as discrete argv entries instead of being embedded into command strings.

• Regression coverage

  • Added/updated focused tests around affected execution paths to assert safe command formation and guard against regressions in command invocation behavior.

// Before (unsafe pattern)
cmd := exec.Command("sh", "-c", fmt.Sprintf("tool --flag %s", userValue))

// After (safe pattern)
cmd := exec.Command("tool", "--flag", userValue)

• Fixes [sighthound] Security findings in github/gh-aw #50494