fix(shellwrap): quote arguments containing # so the login shell keeps them (#1756)

## Bug

Stdio upstreams are launched as `$SHELL -l -c "<joined command>"`
(`shellwrap.WrapWithUserShell`). Each argument goes through
`Shellescape`, but the POSIX branch only quotes when the argument
contains one of `" \t\n\r\"'\\$`;&|<>(){}[]?*~"`, and `#` is not in that
set. A word that begins with `#` starts a comment in `sh`/`bash`/`zsh
-c`, so the shell silently drops that argument and everything after it.

## Trigger

```json
{"name": "slack", "command": "npx", "args": ["-y", "slack-mcp", "#general", "--port", "3000"]}
```

The shell runs `npx -y slack-mcp`. `#general --port 3000` never reaches
the child.

## Fix

Add `#` to the POSIX special-character set, so such arguments are
single-quoted like every other metacharacter. The POSIX escaper copy in
`internal/oauth/spawnargv_round2_test.go` is updated to match, since
`TestPosixShellescape_MatchesProduction` pins the two together.

## Test

`TestShellescape_HashSurvivesShell` checks that `#general` is quoted,
then runs `sh -c "printf '%s\n' <escaped args>"` and asserts that all
four arguments (`#general --port 3000 a#b`) come back unchanged.

## Commands run

- `CGO_ENABLED=0 go test -count=1 ./internal/shellwrap`: ok
- `CGO_ENABLED=0 go test -count=1 ./internal/oauth`: ok
- `CGO_ENABLED=0 go test -count=1 -run 'Shell|Wrap|Escape|Docker'
./internal/upstream/core`: ok
- With the fix reverted (`git stash` of `shellwrap.go`), `-run
HashSurvives` fails: it expected `'#general'` but got `#general`, and
the shell output came back as `[""]`.
- `gofmt -l` on the edited files: no output

Author: rudycelekli

internal/oauth/spawnargv_round2_test.go

@@ -30,7 +30,7 @@ func posixShellescape(s string) string {
 	if s == "" {
 		return "''"
 	}
-	if !strings.ContainsAny(s, " \t\n\r\"'\\$`;&|<>(){}[]?*~") {
+	if !strings.ContainsAny(s, " \t\n\r\"'\\$`;&|<>(){}[]?*~#") {
 		return s
 	}
 	return "'" + strings.ReplaceAll(s, "'", "'\"'\"'") + "'"

internal/shellwrap/shellescape_hash_test.go

@@ -0,0 +1,29 @@
+package shellwrap
+
+import (
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// A word that starts with '#' begins a comment in `sh -c`, so an unquoted
+// "#general" would silently drop it and every argument after it.
+func TestShellescape_HashSurvivesShell(t *testing.T) {
+	args := []string{"#general", "--port", "3000", "a#b"}
+	assert.Equal(t, "'#general'", shellescapeFor("#general", false))
+
+	sh, err := exec.LookPath("sh")
+	if err != nil {
+		t.Skip("no POSIX sh on PATH")
+	}
+	parts := []string{"printf", "'%s\\n'"}
+	for _, a := range args {
+		parts = append(parts, shellescapeFor(a, false))
+	}
+	out, err := exec.Command(sh, "-c", strings.Join(parts, " ")).Output()
+	require.NoError(t, err)
+	assert.Equal(t, args, strings.Split(strings.TrimSuffix(string(out), "\n"), "\n"))
+}

internal/shellwrap/shellwrap.go

@@ -73,7 +73,7 @@ func shellescapeFor(s string, windowsStyle bool) string {
 	}
 
 	// Unix shell special characters: if none present, return as-is.
-	if !strings.ContainsAny(s, " \t\n\r\"'\\$`;&|<>(){}[]?*~") {
+	if !strings.ContainsAny(s, " \t\n\r\"'\\$`;&|<>(){}[]?*~#") {
 		return s
 	}
 	// Use single quotes and escape any embedded single quotes.