docs: note jj fallback in repo inference review convention (#327)

REVIEW.md told reviewers that new commands accepting a `repo` positional
should fall back to inferring `owner/repo` from the git remote. After jj
support was added to `src/utils/vcs.rs`, this convention is stale — a
contributor following it would omit the jj fallback and write code
inconsistent with the new pattern (and with the "git or jj remote
(origin)" help text now advertised by `detail bugs`/`detail
rules`/`detail scans`). Updated to mention jj and point at
`src/utils/vcs.rs` for the canonical implementation.

Introduced by commit 9b815d7 (@dbt,
#324)

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<!-- This is an auto-generated description by cubic. -->
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## Summary by cubic
Update REVIEW.md to state that repo inference should fall back to the
git or jj remote and point to `src/utils/vcs.rs` for the canonical
logic. Aligns review guidance with CLI help for `detail bugs`, `detail
rules`, and `detail scans`.

<sup>Written for commit 6e5fac9.
Summary will update on new commits.</sup>

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Co-authored-by: detail-app[bot] <180357370+detail-app[bot]@users.noreply.github.com>

Author: detail-app[bot]

REVIEW.md

@@ -15,7 +15,8 @@ Conventions that require human judgment during review. Lint-enforced rules
 - `--format json` must produce valid JSON. Never mix human-readable text (hints,
   update notices) into JSON output — guard behind a format check.
 - New commands that accept a `repo` positional should make it optional and fall
-  back to inferring `owner/repo` from the git remote.
+  back to inferring `owner/repo` from the git or jj remote (see
+  `src/utils/vcs.rs`).
 
 ## Generated code