One set of mail variables for every mail, and {link} in state mails

{site} and {site_name} everywhere; a state mail adds {self_link} (the
submitter's own page, when their record's state allows a move there),
{desk_link} and {link}, which picks by who the mail is to. gullberg's
{link} mails went out blank until now. Documented in docs/mail.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MznPTC31w5mSUK4XMehcpi

docs/state-machines.md

@@ -57,7 +57,9 @@ a record arrives in it.
 ```
 
 `{field}` is any field of the record, plus `{site}`, `{chain}`,
-`{email}`, `{bucket}` and `{state}`. The body is markdown; it is sent
+`{email}`, `{bucket}`, `{state}` and `{link}` - the page where the
+reader acts on the record, their own or the desk's (every variable is
+in [Mail](mail.md#variables)). The body is markdown; it is sent
 as text with an HTML alternative. `to: submitter` means the record's
 `email` field, and the lint then requires every form filling that
 bucket to collect a required `email`.