fix: read body from stdin when --tags or template is set

[shichenshuo-star]

Problem

obscli note create <name> --tags a,b silently drops the note body when the
content is piped via stdin. The -c/--content help says "(reads from stdin if
not provided)", but with --tags the body never appears in the created file.

Reproduction

mkdir -p /tmp/v/.obsidian
printf '# Hello\n\nbody\n' | obscli --vault /tmp/v note create "t" --tags a,b
cat /tmp/v/t.md
# Only frontmatter is written; the body is gone.

Root cause

In src/plugins/files.rs, cmd_create reads stdin only when note_content.is_empty():

} else if note_content.is_empty() {
    let mut buffer = String::new();
    if !is_terminal::is_terminal(io::stdin()) {
        io::stdin().read_to_string(&mut buffer)?;
        note_content = buffer;
    }
}

But when --tags (or -t/--template) is provided, frontmatter is written into
note_content before this block runs, so the guard is false and stdin is
never read — the body is silently dropped.

Fix

Read stdin whenever -c/--content is not provided and stdin is not a TTY,
appending the body after any existing frontmatter:

} else if !is_terminal::is_terminal(io::stdin()) {
    let mut buffer = String::new();
    io::stdin().read_to_string(&mut buffer)?;
    note_content.push_str(&buffer);
}

Verification

Tested against a local build (debug):

• --tags a,b + stdin → frontmatter + full body ✅
• stdin without tags → body ✅ (no regression)
• -c + --tags → frontmatter + body ✅ (no regression)