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/completion_test.go

@@ -27,7 +27,7 @@ func completionTestEnv(t *testing.T) string {
 			ID: "t", Slug: slug, Title: slug,
 			CreatedAt: now, UpdatedAt: now,
 			Status: status, Category: category, Type: taskType,
-			Mode: "local", Agent: "claude",
+			Mode: "local", Agent: workspace.AgentSpec{Type: "claude"},
 		}
 		if status == "archived" {
 			meta.ArchivedAt = &now