docs(readme): document the one-line update command - #626

ZETETIC-REVIEW: APPROVE

Verified:

1. Diff scope: git diff origin/main...HEAD --stat touches only README.md, 6 insertions, 0 deletions. Pure addition, no reflow of surrounding lines.

2. Command syntax: claude plugin update --help confirms usage claude plugin update [options] <plugin>. The added line claude plugin update hypermnesia-mcp matches this shape. hypermnesia-mcp is a valid plugin name for this shape: it is the same bare name already used one line above in the existing claude plugin install hypermnesia-mcp command, and claude plugin list shows it installed (as hypermnesia-mcp@cortex-plugins, the qualified form; the bare name is the convention this README already uses for install/update).

3. python3 scripts/check_doc_claims.py passes: "doc claims OK (1 declared not-a-claim exemption(s))".

4. gh pr checks 626 -R cdeust/Cortex snapshot at time of review: all applicable checks pass or are path-skipped (this PR only touches README.md, so build/test/docker/fuzz jobs correctly skip). Test (SQLite backend) is pending, not failing. No failing checks observed. This is a snapshot, not polled to completion.

5. Read the Install section in full context. grep -n "plugin update" README.md returns exactly one hit, the new line, so the PR's claim that the update command was previously undocumented holds and the new line does not duplicate an existing one. The new line sits directly after the install block and reads naturally ("Update the same way, one command"). It does not contradict the "Upgrading from an older plugin identity" collapsible section below, which is scoped to the one-time cortex to hypermnesia-mcp rename migration (uninstall/reinstall under the new name), a distinct case from routine plugin update.

Non-blocking note: claude plugin update --help states a restart is required to apply the update; the added README line doesn't mention this. Not a contradiction and outside the stated scope of this review, just flagging for a possible future follow-up.