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/info.go

@@ -43,7 +43,7 @@ func runInfo(cmd *cobra.Command, args []string) error {
 	fmt.Printf("Category:  %s\n", m.Category)
 	fmt.Printf("Status:    %s\n", m.Status)
 	fmt.Printf("Mode:      %s\n", m.Mode)
-	fmt.Printf("Agent:     %s\n", agentLineWithSource(m.Agent, resolver))
+	fmt.Printf("Agent:     %s\n", agentLineWithSource(m.Agent.Type, resolver))
 	fmt.Printf("Launch session mode: %s (%s)\n",
 		resolver.SessionMode().Value, infoSourceLabel(resolver.SessionMode()))
 	// v0.5.1: display timestamps in local time. task.yaml stores UTC;