chore: repoint remaining links at git.swaits.com

Rewrites lingering github.com / *.sr.ht references to this Gitea instance,
translating GitHub and Sourcehut URL shapes to Gitea's (/blob/<ref>/ and
sr.ht /tree/<ref>/item/ -> /src/branch/<ref>/, /tree/<tag> -> /src/tag/<tag>,
/actions/workflows/X -> /actions?workflow=X, /edit/ -> /_edit/).

Left pointing at the old hosts on purpose: pull-request links (PRs were not
migrated), forks that were never migrated, and GitHub-only surfaces Gitea
lacks. OpenPGP identity proofs are untouched by design.

.github/ISSUE_TEMPLATE/config.yml

@@ -1,8 +1,8 @@
 blank_issues_enabled: false
 contact_links:
   - name: Security vulnerability
-    url: https://github.com/swaits/heuropt/security/advisories/new
-    about: Please use private vulnerability reporting — do not open a public issue. See SECURITY.md.
+    url: https://git.swaits.com/swaits/heuropt/src/branch/main/SECURITY.md
+    about: Do not open a public issue. Email steve@waits.net — see SECURITY.md.
   - name: Question / discussion
-    url: https://github.com/swaits/heuropt/discussions
-    about: For open-ended questions or design discussions.
+    url: https://git.swaits.com/swaits/heuropt/issues
+    about: For open-ended questions or design discussions, open an issue and label it "question".

SECURITY.md

@@ -17,14 +17,15 @@ minor versions.
 
 ## Reporting a vulnerability
 
-Please **do not** open a public GitHub issue for a security bug.
-Instead use one of these channels:
+Please **do not** open a public issue for a security bug.
+Instead:
 
-- GitHub's [private vulnerability reporting](https://github.com/swaits/heuropt/security/advisories/new)
-  on the repository.
 - Email **steve@waits.net** with subject line `[heuropt security]
   <short summary>`.
 
+(This project is hosted on [git.swaits.com](https://git.swaits.com/swaits/heuropt),
+which has no private-advisory feature, so email is the reporting channel.)
+
 Please include:
 
 1. A description of the vulnerability and the affected versions.