feat(v0.6): --agent flag on ctask new selects agent type

Resolver gains SetCLIFlagAgent; DefaultAgent now layers CLIFlag above
EnvVar so doctor/info attribution renders the correct precedence chain
(CLIFlag overrides EnvVar overrides ConfigFile overrides Builtin).

ctask new --agent <type> writes agent.type into the new workspace's
task.yaml. Resolution and validation run before workspace.Create, so
--agent custom without a companion command refuses ("type custom
requires command") with no half-created workspace left on disk. The
deferred Phase 1 test TestCLIFlagOverridesEnvVar lands here.

--agent on resume/last/attach is unchanged (one-shot agent.command
override on the AgentSpec — Open Q 1).

internal/config/resolver.go

@@ -79,6 +79,24 @@ type Resolver struct {
 	envAgent       string
 	envSessionMode string
 	envEditor      string
+
+	// cliFlagAgent, when non-empty, is the value of the --agent CLI flag.
+	// Set by SetCLIFlagAgent after Cobra parses flags; consulted by
+	// DefaultAgent as the highest-priority layer (SettingSource.CLIFlag).
+	cliFlagAgent string
+}
+
+// SetCLIFlagAgent records the value of the --agent CLI flag for
+// resolution. Cmd-layer code calls this AFTER Cobra parses flags, so the
+// flag value can participate in source attribution. Empty string means
+// the flag was not passed; the env-var layer is consulted instead.
+//
+// Currently used only by `ctask new` (where --agent is a type selector,
+// per v0.6 spec §5 + Open Q 1). Other commands' --agent flags act as
+// one-shot agent.command overrides on the AgentSpec, NOT as resolver
+// inputs — they do not call this setter.
+func (r *Resolver) SetCLIFlagAgent(value string) {
+	r.cliFlagAgent = value
 }
 
 // configPathForTest, when non-empty, overrides ConfigFilePath() for
@@ -223,14 +241,28 @@ func (r *Resolver) SeedDir() ResolvedSetting {
 		defaultSeedDir("seed"), cfgVal, r.envSeedDir, expandPath)
 }
 
-// DefaultAgent resolves the default agent command.
+// DefaultAgent resolves the default agent command. Layering is
+// Builtin → ConfigFile → EnvVar → CLIFlag: when SetCLIFlagAgent has
+// recorded a non-empty --agent value, it wins and the previously
+// resolved setting is chained as Override so doctor/info can render the
+// full precedence path.
 func (r *Resolver) DefaultAgent() ResolvedSetting {
 	cfgVal := ""
 	if r.cfg != nil {
 		cfgVal = r.cfg.DefaultAgent
 	}
-	return r.stringSetting("default_agent", "CTASK_AGENT",
+	base := r.stringSetting("default_agent", "CTASK_AGENT",
 		"claude", cfgVal, r.envAgent, nil)
+	if r.cliFlagAgent != "" {
+		prev := base
+		return ResolvedSetting{
+			Key:      "default_agent",
+			Value:    r.cliFlagAgent,
+			Source:   CLIFlag,
+			Override: &prev,
+		}
+	}
+	return base
 }
 
 // DefaultCategory resolves the default category for new workspaces.