fix(comment): don't block on stdin when body is given positionally (#1011)

## The bug

`tea comment <idx> "body"` hangs forever in any non-TTY context — CI pipelines, subshells, agent harnesses, scripts — unless the user explicitly redirects stdin with `< /dev/null`.

### Reproduction (released `tea` 0.14.1)

```
# Plain positional body — hangs forever
$ tea comment --repo owner/repo 1 "body text"

# Heredoc body — hangs forever
$ tea comment --repo owner/repo 1 "$(cat <<'EOM'
body text
EOM
)"

# Workaround that works (but shouldn't be necessary)
$ tea comment --repo owner/repo 1 "body text" < /dev/null
```

Verified on `gitea.com` (server 1.26.0+dev) with `tea 0.14.1`. Both hanging cases hit a 30s timeout in testing; in normal shell use they hang indefinitely.

## Root cause

`cmd/comment.go:58-65`:

```go
body := strings.Join(ctx.Args().Tail(), " ")
if interact.IsStdinPiped() {
    if bodyStdin, err := io.ReadAll(ctx.Reader); err != nil {
        return err
    } else if len(bodyStdin) != 0 {
        body = strings.Join([]string{body, string(bodyStdin)}, "\n\n")
    }
}
```

`interact.IsStdinPiped()` is implemented as `!term.IsTerminal(os.Stdin)` — true for *any* non-TTY stdin, not just for piped data. When tea enters this branch in a subshell where stdin is open but no producer ever writes to it (the typical case for shell scripts and automation), `io.ReadAll` blocks waiting for an EOF that never arrives.

## Fix

Only consume stdin when the user did **not** supply a positional body. If they passed a body via args, that's their content — ignore stdin entirely.

```go
if len(body) == 0 && interact.IsStdinPiped() {
    // ... read stdin ...
    body = string(bodyStdin)
}
```

## Behavior matrix

| Invocation | Before | After |
|---|---|---|
| `tea comment <idx> "body"` (TTY) | works | works |
| `tea comment <idx> "body"` (non-TTY, no redirect) | **hangs forever** | **works** |
| `tea comment <idx> "body" < /dev/null` | works | works |
| `echo body \| tea comment <idx>` | works | works |
| `tea comment <idx> "prefix"` + piped stdin | "prefix" + stdin concatenated | positional body wins, stdin ignored |
| `tea comment <idx>` (TTY, no body, no pipe) | opens editor | opens editor |

The one behavior change is the prefix-plus-stdin case (5th row). I couldn't find anyone relying on that pattern and it wasn't documented; defaulting to "positional body wins" matches the principle of least surprise.

## Verification

Confirmed each row against `dinsmoor/tea-testing` issue #1 on `gitea.com` with the patched binary. Previously-hanging invocations now post in <0.5s. Piped-stdin path unchanged.

---

This patch was authored interactively with an AI assistant, driven and reviewed by a human (Tyler / @dinsmoor) every step.

*pull request created by Tyler's lovingly wrangled demon machine <3*

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/1011
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Tyler <tyler@dinsmoor.us>
Co-committed-by: Tyler <tyler@dinsmoor.us>

cmd/comment.go

@@ -56,12 +56,16 @@ func runAddComment(requestCtx stdctx.Context, cmd *cli.Command) error {
 	}
 
 	body := strings.Join(ctx.Args().Tail(), " ")
-	if interact.IsStdinPiped() {
+	// Only consume stdin if no positional body was given. interact.IsStdinPiped()
+	// is true for any non-TTY stdin (CI, subshells, agent harnesses) — not just
+	// piped data — so reading unconditionally would block forever in those
+	// contexts when the body is supplied via args.
+	if len(body) == 0 && interact.IsStdinPiped() {
 		// custom solution until https://github.com/AlecAivazis/survey/issues/328 is fixed
 		if bodyStdin, err := io.ReadAll(ctx.Reader); err != nil {
 			return err
 		} else if len(bodyStdin) != 0 {
-			body = strings.Join([]string{body, string(bodyStdin)}, "\n\n")
+			body = string(bodyStdin)
 		}
 	} else if len(body) == 0 {
 		if err := huh.NewForm(