`--stdin` support — read YAML request from stdin

[xaaha]

Summary

Allow hulak to read a YAML request definition from stdin instead of requiring a file on disk.

Problem

Today, every request requires a .hk.yaml file. For one-off requests or automation, this means creating a temp file, running hulak, then cleaning up:

cat > /tmp/req.hk.yaml << 'EOF'
method: POST
url: https://api.example.com/users
body:
  raw: '{"name": "John"}'
EOF
hulak -fp /tmp/req.hk.yaml
rm /tmp/req.hk.yaml

Proposed behavior

# Pipe YAML directly
echo 'method: GET
url: https://httpbin.org/get' | hulak --stdin

# Heredoc for multi-line
hulak --stdin << 'EOF'
method: POST
url: https://api.example.com/users
headers:
  Content-Type: application/json
body:
  raw: '{"name": "John"}'
EOF

# Combine with --quiet for full automation
cat request.yaml | hulak --stdin --quiet | jq '.response.body'

When --stdin is set:

• Read YAML from stdin instead of a file
• Parse it through the same yamlparser pipeline as file-based requests
• -env flag still works for template variable resolution
• No _response.json file is saved (no source file to derive the name from), response goes to stdout only

Implementation notes

• The YAML parsing path already exists in pkg/yamlparser/ — this just needs a new input source
• Detect stdin via os.Stdin stat or the --stdin flag
• Template actions like {{.token}} should still resolve from env files
• Actions like {{getValueOf ...}} and {{getFile ...}} should still work (they reference the filesystem, not the input source)

Parent

Part of #154 — v0.3 ergonomics