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/open.go

@@ -51,7 +51,7 @@ func runOpen(cmd *cobra.Command, args []string) error {
 		WsPath:      ws.Path,
 		WsRoot:      ws.Root,
 		WsMeta:      ws.Meta,
-		Agent:       ws.Meta.Agent,
+		Agent:       ws.Meta.Agent.Type,
 		Shell:       true, // open always launches a shell
 		Force:       openForce,
 		Direct:      openDirect,