fix(cli/mcp): prevent command injection via $EDITOR/$VISUAL

[YoungDan-hero]

Summary

mcpEditConfigLaunchCommand concatenated the raw $VISUAL/$EDITOR value
into a sh -lc "<editor> <path>" command string. Only the path was
shell-quoted, so an editor value like vim; rm -rf ~ (or any environment
pollution) would execute arbitrary shell commands.

Severity

P1 (command injection). Exploitation requires the environment variable to be
set to a malicious value — not remote input, but a real attack surface for
shared accounts, compromised tooling, or untrusted config sources.

Fix

Replace the sh -lc construction with editorLaunchCmd:

1. Expand $VAR references via os.ExpandEnv (no shell)
2. Split into argv via strings.Fields (supports code --wait)
3. Expand leading ~ / ~/ via os.UserHomeDir (no shell)
4. exec.Command(args[0], append(args[1:], path)...) — binary resolved by OS

Shell metacharacters become literal argv tokens and cannot be executed. This
matches the safe pattern already used by the terminal-editor fallback
(exec.Command(bin, path)) in the same function.

The now-unused shellQuote helper is removed.

Behavior preservation

EDITOR value	Prior (sh -lc)	New (exec)
vim	✅	✅
code --wait	✅	✅ (Fields split)
$HOME/bin/x	✅ (shell expand)	✅ (ExpandEnv)
~/bin/x	✅ (shell expand)	✅ (tilde expand)
~	✅ (shell expand)	✅ (tilde expand)
vim; rm -rf ~	❌ executes rm	✅ safe (literal)

No behavior change for legitimate values. Shell quoting inside the editor
value (e.g. 'my editor') is not supported, but was already broken under
the prior sh -lc path for non-single-quoted values, and no common
EDITOR/VISUAL value requires it.

Test (7 cases)

• TestMCPEditConfigLaunchUsesVisualBeforeEditor — direct invocation
• TestMCPEditConfigLaunchEditorWithArgs — code --wait arg splitting
• TestMCPEditConfigLaunchEditorRejectsShellMetachars — injection defense
• TestMCPEditConfigLaunchEditorExpandsEnvVar — $VAR expansion
• TestMCPEditConfigLaunchEditorExpandsTilde — ~ / ~/ expansion
• TestMCPEditConfigLaunchEditorTildeNotInPayload — tilde not abused
• TestMCPEditConfigLaunchFallsBackToTerminalEditor — fallback unchanged

Verification

• go build ./... — pass
• go test ./internal/cli/ — all pass
• go vet ./internal/cli/... — clean
• gofmt -l — clean

Cache-impact: none