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/new.go

@@ -92,7 +92,7 @@ func runNew(cmd *cobra.Command, args []string) error {
 		Title:           title,
 		Category:        category,
 		Mode:            "local",
-		Agent:           agent,
+		AgentSpec:       workspace.AgentSpec{Type: agent},
 		IsProject:       newProject,
 		SeedDir:         config.ResolveSeedDir(),
 		SkipCategoryDir: skipCategoryDir,