feat: Embedded runbook expressions (#54)

Expands existing behaviour by requiring either `path` or `expr` in `exec
block with optional `runtime` argument, passing the expression via
stdin. If `runtime` isn't present the default to `sh`.
```
- type: exec
  exec:
    expr: [...]
    runtime: [...]
```

Author: samgaw

internal/pkg/runbook/exec.go

@@ -3,30 +3,36 @@ package runbook
 import (
 	"bytes"
 	"context"
-	"encoding/json"
 	"errors"
 	"os"
 	"os/exec"
+	"strings"
 	"text/template"
 )
 
 type execConfig struct {
-	Path string   `yaml:"path"`
-	Args []string `yaml:"args"`
+	Path    string   `yaml:"path"`
+	Expr    string   `yaml:"expr"`
+	Runtime string   `yaml:"runtime"` // optional, default to "sh -s"
+	Args    []string `yaml:"args"`
 }
 
 type execAction struct {
 	cfg execConfig
 }
 
 func newExecAction(cfg execConfig) (Action, error) {
-	if cfg.Path == "" {
-		return nil, errors.New("exec.path is required")
+	if cfg.Path == "" && cfg.Expr == "" {
+		return nil, errors.New("either exec.path or exec.expr is required")
+	}
+	if cfg.Path != "" && cfg.Expr != "" {
+		return nil, errors.New("exec.path and exec.expr are mutually exclusive")
 	}
 	return &execAction{cfg: cfg}, nil
 }
 
 func (e *execAction) Execute(ctx context.Context, cre map[string]any) error {
+	// Template substitution for args
 	args := make([]string, len(e.cfg.Args))
 	for i, a := range e.cfg.Args {
 		tmpl, err := template.New("arg").Funcs(funcMap()).Parse(a)
@@ -38,14 +44,48 @@ func (e *execAction) Execute(ctx context.Context, cre map[string]any) error {
 		}
 	}
 
-	raw, err := json.Marshal(cre)
-	if err != nil {
-		return err
+	var cmd *exec.Cmd
+
+	switch {
+	// External command with args
+	case e.cfg.Path != "":
+		cmd = exec.CommandContext(ctx, e.cfg.Path, args...)
+
+	// expr + runtime piped via stdin
+	case e.cfg.Expr != "":
+  	// Expand template variables
+  	expr, err := renderTemplate(e.cfg.Expr, cre)
+ 		if err != nil {
+			return err
+		}
+
+		runtime := e.cfg.Runtime
+		if runtime == "" {
+			runtime = "sh -s"
+		}
+
+		parts := splitRuntime(runtime)
+		cmd = exec.CommandContext(ctx, parts[0], append(parts[1:], args...)...)
+    cmd.Stdin = strings.NewReader(expr)
 	}
 
-	cmd := exec.CommandContext(ctx, e.cfg.Path, args...)
-	cmd.Stdin = bytes.NewReader(raw)
+	// Common output wiring
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
+
 	return cmd.Run()
 }
+
+func renderTemplate(input string, data map[string]any) (string, error) {
+	tmpl, err := template.New("inline").Funcs(funcMap()).Parse(input)
+	if err != nil {
+		return "", err
+	}
+	var buf bytes.Buffer
+	err = tmpl.Execute(&buf, data)
+	return buf.String(), err
+}
+
+func splitRuntime(runtime string) []string {
+	return strings.Fields(runtime) // basic split
+}

internal/pkg/runbook/runbook.go

@@ -29,6 +29,9 @@ actions:
     regex: "CRE-2025-0025"
     exec:
       path: ./action.sh
+      expr: |
+        echo "Critical incident: {{ field .cre "Id" }}"
+      runtime: bash -
       args:
         - '{{ field .cre "Id" }}'
         - '{{ len .hits }}'