feat(v0.6): AgentSpec field on TaskMeta with backward-compat unmarshal

Replace TaskMeta.Agent (string) with TaskMeta.Agent (AgentSpec) carrying
type/command/args/env. Custom UnmarshalYAML preserves the legacy scalar
form: a built-in name (claude, opencode) maps to that type; any other
scalar maps to type=custom with the scalar as command. A missing agent
field leaves Type empty so the resolver fills in default_agent at launch.

ValidateAgentSpec enforces: known type (claude|opencode|custom),
type=custom requires command, command must be an executable name or
path with no whitespace or shell metacharacters.

Launch-path wiring (Task 3) and the --agent flag rework (Task 4) are
intentionally not part of this commit; cmd/* call sites are patched to
the minimum needed for the build to compile.

cmd/archive_test.go

@@ -22,16 +22,16 @@ func makeArchiveWs(t *testing.T, root, category, dirName string) string {
 	now := time.Now().UTC().Truncate(time.Second)
 	slug := dirName[11:]
 	meta := &workspace.TaskMeta{
-		ID:            "test",
-		Slug:          slug,
-		Title:         slug,
-		CreatedAt:     now,
-		UpdatedAt:     now,
-		Status:        "active",
-		Category:      category,
-		Type:          "task",
-		Mode:          "local",
-		Agent:         "claude",
+		ID:        "test",
+		Slug:      slug,
+		Title:     slug,
+		CreatedAt: now,
+		UpdatedAt: now,
+		Status:    "active",
+		Category:  category,
+		Type:      "task",
+		Mode:      "local",
+		Agent:     workspace.AgentSpec{Type: "claude"},
 	}
 	workspace.WriteMeta(filepath.Join(dir, "task.yaml"), meta)
 	return dir