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.

internal/session/adopt_test.go

@@ -31,7 +31,7 @@ func newAdoptionFixture(t *testing.T) *adoptionFixture {
 		ID: "demo-id", Slug: "demo", Title: "demo",
 		CreatedAt: initial, UpdatedAt: initial,
 		Status: "active", Category: "projects", Type: "project",
-		Mode: "local", Agent: "claude",
+		Mode: "local", Agent: workspace.AgentSpec{Type: "claude"},
 	}
 	if err := workspace.WriteMeta(filepath.Join(ws, "task.yaml"), meta); err != nil {
 		t.Fatalf("WriteMeta: %v", err)