Only fire the routine when an issue/PR is assigned to Claude

Gitea's X-Gitea-Event only gives the coarse event; the precise
"assigned" sub-event lives in X-Gitea-Event-Type (issue_assign,
pull_request_assign). Read that header and check the assignees list
against CLAUDE_GITEA_USERNAME so unrelated assignment changes get a
200 "ignored" instead of triggering a routine fire. Document the known
Gitea limitation where it can't tell us who was just added to
assignees, only the full current list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -21,3 +21,8 @@ DEDUP_TTL_MS=300000
 # Timeout in milliseconds for the call to the Anthropic API (prevents it from
 # hanging indefinitely if there's no outbound internet access)
 FIRE_TIMEOUT_MS=10000
+
+# Gitea username to match against issue/PR assignees for "issue_assign" and
+# "pull_request_assign" events. Only deliveries assigning this user forward
+# a routine fire; other assignment changes are ignored (200 "ignored").
+CLAUDE_GITEA_USERNAME=claude