feat(v0.6): ctask agents check + doctor integration

ctask agents check [workspace] validates that a workspace's agent
configuration can be launched without launching it: agent type known,
command resolvable on PATH, launch_dir valid, AGENTS.md present,
CLAUDE.md shim present (WARN only, claude type only). agent.env keys
are displayed informationally, with a WARN line when any key shadows a
ctask-exported CTASK_* var. Returns non-zero when any check FAILs.

ctask doctor includes the same sweep when a workspace context is
resolvable — the most-recently-active workspace via
workspace.MostRecentActive. When no active workspace exists, doctor
shows "Agent check: skipped (no workspace context)" without bumping
the failure counter. runAgentsCheckOnWorkspace is shared between the
standalone command and the doctor integration.

TestCompletionSubcommandViaExecute is made order-independent: cobra's
default completion command captures the root output writer on the first
Execute() in the process, and the new agents-check tests now run an
Execute() earlier in the suite.

cmd/completion_test.go

@@ -197,6 +197,17 @@ func TestCompletionSubcommandViaExecute(t *testing.T) {
 	defer rootCmd.SetOut(os.Stdout)
 	defer rootCmd.SetErr(os.Stderr)
 
+	// Cobra's default `completion` command captures the root's output
+	// writer once, when it is first created on the first Execute() anywhere
+	// in the process (see InitDefaultCompletionCmd). Drop any previously
+	// created instance so the Execute() below re-creates it bound to buf —
+	// keeps this test independent of which other test ran Execute() first.
+	for _, c := range rootCmd.Commands() {
+		if c.Name() == "completion" {
+			rootCmd.RemoveCommand(c)
+		}
+	}
+
 	rootCmd.SetArgs([]string{"completion", "bash"})
 	defer rootCmd.SetArgs(nil)
 	if err := rootCmd.Execute(); err != nil {