feat(v0.6): launch path carries ResolvedAgent (command + args + env)

LaunchOpts.Agent (string) and WorkspaceEntryOptions.Agent (string) are
replaced by *agent.Resolved, carrying Command, Args, and Env. The five
entry commands (new, resume, last, open, attach) each construct an
AgentSpec from the workspace metadata, apply --agent as a one-shot
agent.command override (Open Q 1 — keeps muscle memory for users
passing executable paths), call agent.Resolve, and pass the result
through. resolveEntryAgent centralises the resume/last/open/attach path.

shell.ExecAgent and shell.ExecTmuxAgent gain an args parameter; agent.env
is merged into the env map at the session.Run launch switch, AFTER
ctask's exported CTASK_* vars (per spec §5: agent.env wins on collision).
mergeAgentEnv is the centralised merge.

Lease, manifest, write lock, heartbeat, summary, and provisional
cleanup are unchanged. The Agent string fields on Lease, SessionSummary,
and SessionInfo continue to record the launched command for diagnostics.

cmd/attach.go

@@ -43,16 +43,16 @@ func runAttach(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("updating metadata: %w", err)
 	}
 
-	agent := attachAgent
-	if agent == "" {
-		agent = ws.Meta.Agent.Type
+	resolved, err := resolveEntryAgent(ws.Meta.Agent, attachAgent)
+	if err != nil {
+		return err
 	}
 
 	return runWorkspaceEntry(WorkspaceEntryOptions{
 		WsPath:           ws.Path,
 		WsRoot:           ws.Root,
 		WsMeta:           ws.Meta,
-		Agent:            agent,
+		ResolvedAgent:    resolved,
 		Shell:            false, // attach defaults to agent
 		Force:            attachForce,
 		AlwaysPersistent: true, // attach is always tmux, regardless of env