fix(cli): use picoclaw home dir for history file instead of temp dir

Security fix for issue #2234:

- HistoryFile was hardcoded to os.TempDir() (/tmp on Linux)
- /tmp is typically world-readable (mode 1777) exposing command history
- An attacker could create symlinks to overwrite sensitive files
- Now uses internal.GetPicoclawHome() which respects $PICOCLAW_HOME
  environment variable and falls back to ~/.picoclaw

cmd/picoclaw/internal/agent/helpers.go

@@ -84,7 +84,7 @@ func interactiveMode(agentLoop *agent.AgentLoop, sessionKey string) {
 
 	rl, err := readline.NewEx(&readline.Config{
 		Prompt:          prompt,
-		HistoryFile:     filepath.Join(os.TempDir(), ".picoclaw_history"),
+		HistoryFile:     filepath.Join(internal.GetPicoclawHome(), ".picoclaw_history"),
 		HistoryLimit:    100,
 		InterruptPrompt: "^C",
 		EOFPrompt:       "exit",