test(cli): prove a session private key never reaches any output

### Description

No test proves a session private key never reaches stdout, stderr, the logger, telemetry, or the rerun hint. The key is copied into `process.env.SESSION_KEY` by auto-load and by `--credentials-file`, and the upload paths log raw error objects. Today's guarantee rests on reading the code, not on a check that fails when someone adds a log line.

### Impact

A regression would write a private key into CI logs, agent transcripts, or Sentry. Nobody would notice until a key was misused.

### Expected behavior

A unit test that plants a sentinel private key through each credential path (`SESSION_KEY` env, `--credentials-file`, the saved session file) and drives the paths that print the most: `login` with a saved session, `balance`, `add` failing its funds preflight, `import` failing its preflight, an RPC error with `--verbose`, and a telemetry flush failure. The test captures every log sink and fails if the sentinel, or any 64-hex string, appears.

### Additional context

Raised in the Sep 8 safety review of the login stack (#699 to #703), which added the redacted rerun hint and the credentials-file allowlist. Related: #697, #711.