Add session discovery and template support

internal/config/load.go

@@ -5,6 +5,8 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"regexp"
+	"strings"
 
 	"gopkg.in/yaml.v3"
 )
@@ -21,21 +23,56 @@ func LoadPipeline(path string) (*PipelineConfig, error) {
 
 // LoadSession loads session configuration from a YAML file with strict field checking.
 func LoadSession(path string) (*SessionConfig, error) {
-	var cfg SessionConfig
-	if err := decodeStrictYAML("session", path, &cfg); err != nil {
+	return LoadSessionWithOptions(path, SessionLoadOptions{})
+}
+
+// SessionLoadOptions configures session template rendering behavior.
+type SessionLoadOptions struct {
+	SessionID string
+}
+
+// LoadSessionWithOptions loads session configuration from a YAML file with
+// strict field checking after template rendering.
+func LoadSessionWithOptions(path string, opts SessionLoadOptions) (*SessionConfig, error) {
+	sessionBytes, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("load session config: session file %q: open: %w", path, err)
+	}
+
+	rendered, err := renderSessionTemplate(string(sessionBytes), opts)
+	if err != nil {
 		return nil, fmt.Errorf("load session config: %w", err)
 	}
+
+	var cfg SessionConfig
+	if err := decodeStrictYAMLFromReader("session", path, strings.NewReader(rendered), &cfg); err != nil {
+		return nil, fmt.Errorf("load session config: %w", err)
+	}
+	if strings.TrimSpace(opts.SessionID) != "" && strings.TrimSpace(cfg.SessionID) != "" && strings.TrimSpace(cfg.SessionID) != strings.TrimSpace(opts.SessionID) {
+		return nil, fmt.Errorf(
+			"load session config: session file %q: session_id mismatch: --session-id %q does not match rendered session_id %q",
+			path,
+			strings.TrimSpace(opts.SessionID),
+			strings.TrimSpace(cfg.SessionID),
+		)
+	}
 	return &cfg, nil
 }
 
 // Load loads and resolves combined pipeline and session configuration.
 func Load(pipelinePath, sessionPath string) (*Config, error) {
+	return LoadWithSessionOptions(pipelinePath, sessionPath, SessionLoadOptions{})
+}
+
+// LoadWithSessionOptions loads and resolves combined pipeline and session
+// configuration with session template options.
+func LoadWithSessionOptions(pipelinePath, sessionPath string, sessionOpts SessionLoadOptions) (*Config, error) {
 	pipelineCfg, err := LoadPipeline(pipelinePath)
 	if err != nil {
 		return nil, err
 	}
 
-	sessionCfg, err := LoadSession(sessionPath)
+	sessionCfg, err := LoadSessionWithOptions(sessionPath, sessionOpts)
 	if err != nil {
 		return nil, err
 	}
@@ -55,7 +92,11 @@ func decodeStrictYAML(kind, path string, out any) error {
 	}
 	defer f.Close()
 
-	dec := yaml.NewDecoder(f)
+	return decodeStrictYAMLFromReader(kind, path, f, out)
+}
+
+func decodeStrictYAMLFromReader(kind, path string, r io.Reader, out any) error {
+	dec := yaml.NewDecoder(r)
 	dec.KnownFields(true)
 	if err := dec.Decode(out); err != nil {
 		return fmt.Errorf("%s file %q: strict decode failed: %w", kind, path, err)
@@ -69,6 +110,36 @@ func decodeStrictYAML(kind, path string, out any) error {
 	return nil
 }
 
+var sessionTemplatePattern = regexp.MustCompile(`\{\{\s*([a-zA-Z_][a-zA-Z0-9_]*)\s*\}\}`)
+
+func renderSessionTemplate(content string, opts SessionLoadOptions) (string, error) {
+	sessionID := strings.TrimSpace(opts.SessionID)
+	rendered := content
+	if sessionID != "" {
+		rendered = strings.ReplaceAll(rendered, "{{session_id}}", sessionID)
+		rendered = strings.ReplaceAll(rendered, "{{ session_id }}", sessionID)
+	}
+
+	unresolved := sessionTemplatePattern.FindAllStringSubmatch(rendered, -1)
+	if len(unresolved) > 0 {
+		vars := make([]string, 0, len(unresolved))
+		for _, m := range unresolved {
+			if len(m) > 1 {
+				vars = append(vars, m[1])
+			}
+		}
+		if len(vars) > 0 {
+			return "", fmt.Errorf(
+				"session file template rendering failed: unresolved template variable(s): %s; pass --session-id when using {{ session_id }}",
+				strings.Join(vars, ", "),
+			)
+		}
+		return "", fmt.Errorf("session file template rendering failed: unresolved template placeholders remain")
+	}
+
+	return rendered, nil
+}
+
 func shortName(path, fallback string) string {
 	base := filepath.Base(path)
 	if base == "." || base == string(filepath.Separator) {