fix: target intended users in lgtm approvals (#7023)

Maintainers frequently direct lgtm at tagged users who volunteer on an issue, but approval previously applied to the issue or PR author. Mentioned users are now approval targets, with the author as the fallback.

Prose and examples can also contain lgtm without intending approval. Only treat lgtm or lgtmi at the start, after leading mentions, or at the end as commands. Process PR comments too, where approvals previously had no effect.

CONTRIBUTING.md

@@ -31,7 +31,7 @@ Approval happens through maintainer replies on issues:
 - `lgtmi`: your future issues will not be auto-closed
 - `lgtm`: your future issues and PRs will not be auto-closed
 
-`lgtmi` does not grant rights to submit PRs. Only `lgtm` grants rights to submit PRs.
+The command must be at the start of the reply (optionally after one or more `@username` mentions) or at the end. `lgtmi` does not grant rights to submit PRs. Only `lgtm` grants rights to submit PRs.
 
 ## Quality Bar For Issues
 
@@ -45,7 +45,7 @@ If you open an issue, keep it short, concrete, and worth reading.
 - Explain why it matters.
 - If you want to implement the change yourself, say so.
 
-If the issue is real and written well, a maintainer may reopen it, reply `lgtmi`, or reply `lgtm`.
+If the issue is real and written well, a maintainer may reopen it or reply with `lgtmi` or `lgtm` in the command position described above.
 
 ## Blocking
 
@@ -55,7 +55,7 @@ If you send a large volume of issues through automation, your GitHub account wil
 
 ## Before Submitting a PR
 
-Do not open a PR unless you have already been approved with `lgtm`.
+Do not open a PR unless you have already been approved by a maintainer using `lgtm` in the command position described above.
 
 Before submitting a PR: