#8653: Docs: clarify DM pairing approve command

Clarifies DM pairing approve command placeholder (<code> → <PAIRING_CODE>) Testing: not applicable (docs-only change). <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the README’s DM pairing instructions by replacing the ambiguous `<code>` placeholder with a clearer `<PAIRING_CODE>` token and expanding the parenthetical to explain that approving a pairing adds the sender to the local allowlist so future DMs are processed. This fits within the existing “Security defaults (DM access)” section, improving the usability of the documented CLI command without changing any runtime behavior. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk (docs-only placeholder clarification). - The change is a single-line README clarification that does not affect code paths, builds, or configuration semantics; it improves readability and reduces ambiguity in the documented command usage. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->