v0.7.1

• Restore numeric rendering in generated Markdown: attachment byte sizes and message and attachment counts print as numbers again, not an arbitrary-precision wrapper object, after the afdata dependency enabled that globally.
• Rework config editing to change .afmail/config.json in place through afdata's DocumentFile — validate the whole config before writing, commit atomically, and preserve on-disk key order — replacing the hand-written sparse-diff writer.
• Migrate onto afdata 0.20, which absorbed the retired agent-first-config, adopting its redesigned config, document, version, and help APIs.
• Bump the agent-first-slug dependency to 0.3.0.