docs(contributing): make the issue templates work on the GitHub mirror too

The templates were written against Gitea's schema alone, but the mirror
serves the same .github/ISSUE_TEMPLATE/ files to GitHub, where two of them
would have silently failed:

- **config.yaml -> config.yml.** Gitea accepts either spelling
  (modules/structs/issue.go: `base == "config.yaml" || base == "config.yml"`),
  GitHub only recognizes config.yml. Under the old name the mirror's
  template chooser would have shown neither the contact links nor the
  blank-issue setting.
- **bug.yaml `about:` -> bug.yml `description:`.** GitHub requires
  `description` on a YAML issue form; Gitea's IssueTemplate.About carries
  the comment "Using description in a template file is compatible" and
  falls back to it at modules/issue/template/unmarshal.go:126. So
  `description` is the one spelling both forges accept. The markdown
  templates keep `about:`, which is correct for their format on both.

Both files now carry a comment explaining the constraint, so neither gets
"tidied" back into a broken state.

Also add a contact link pointing at the canonical Gitea tracker, so someone
arriving from the mirror is steered to the right place before they file,
and document mirror parity in CONTRIBUTING.md: the labels must be created
on both sides by hand, since mirroring copies files rather than repository
settings and GitHub drops a labels: entry naming a label it doesn't have.
Note too that GitHub has no exclusive labels, so the one-of rule on
Priority/, Reviewed/, and Status/ holds only by convention there.

.github/ISSUE_TEMPLATE/bug.yml

@@ -1,5 +1,13 @@
+# The key below is `description`, NOT `about`.
+#
+# GitHub requires `description` on a YAML issue form and rejects the
+# template without it; Gitea wants `about` but explicitly accepts
+# `description` as a compatible alias. `description` is therefore the
+# only spelling that works on both the canonical repo and the mirror.
+# The markdown templates beside this one still use `about`, which is
+# correct for their format on both forges.
 name: Bug report
-about: Something in the config is broken or behaves unexpectedly
+description: Something in the config is broken or behaves unexpectedly
 labels:
   - Kind/Bug
 body:

.github/ISSUE_TEMPLATE/config.yml

@@ -1,10 +1,18 @@
-# Gitea reads this alongside the templates in this directory.
+# Read by both Gitea (the canonical repo) and GitHub (the mirror).
+#
+# The .yml extension is required: Gitea accepts config.yaml or config.yml,
+# but GitHub only recognizes config.yml and silently ignores config.yaml.
+# Don't "tidy" this back to .yaml — the chooser on the mirror stops working.
+#
 # Blank issues stay enabled deliberately: the three templates cover bugs,
 # features, and docs, and anything else (a chore, a refactor, a question)
 # is better served by an empty box than by a template that doesn't fit.
 blank_issues_enabled: true
 
 contact_links:
+  - name: Canonical repository and issue tracker
+    url: https://git.rootiest.dev/rootiest/fish-config/issues
+    about: fish-config is developed on Gitea. If you came from the GitHub mirror, please file here instead — the mirror is read-only and issues opened there are easy to miss.
   - name: Contributing guide
     url: https://git.rootiest.dev/rootiest/fish-config/src/branch/main/CONTRIBUTING.md
     about: Branch naming, commit conventions, coding standards, and the label taxonomy.