Add redacted session exports

[jongio]

Export can write complete conversations to Markdown or JSON, but the only secret masking today lives in the preview pane. Add a --redact flag to dispatch export so users can share an export without copying raw tokens from the session store.

Acceptance criteria:

• dispatch export <id> --redact --stdout masks the same common secret patterns as the preview.
• --redact works with Markdown and JSON exports, stdout and file output.
• Existing export behavior stays unchanged unless the flag is set.
• Usage text and README mention the flag.