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/entry_test.go

@@ -65,7 +65,7 @@ func TestRunWorkspaceEntryIsInjectable(t *testing.T) {
 	want := WorkspaceEntryOptions{
 		WsPath:      "/tmp/ws",
 		WsRoot:      "/tmp",
-		WsMeta:      &workspace.TaskMeta{Slug: "demo", Category: "projects", Mode: "local", Agent: "claude"},
+		WsMeta:      &workspace.TaskMeta{Slug: "demo", Category: "projects", Mode: "local", Agent: workspace.AgentSpec{Type: "claude"}},
 		Agent:       "claude",
 		Shell:       true,
 		CommandName: "test",