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.

CONTRIBUTING.md

@@ -56,6 +56,13 @@ refuses to submit without them. The other two are markdown templates in the
 same comment-guided style as `.github/PULL_REQUEST_TEMPLATE.md`, since what
 they ask for is open-ended prose that structure would only get in the way of.
 
+GitHub reads these same files on the mirror, and its schema differs from
+Gitea's in two places, so both are pinned to the spelling that works on both
+and each file says so in a comment: the chooser config must be `config.yml`
+(GitHub ignores `config.yaml`), and `bug.yml` declares `description:` rather
+than `about:` (GitHub requires it; Gitea accepts it as an alias). The two
+markdown templates keep `about:`, which is correct for their format on both.
+
 ### Issue titles
 
 **Issue titles are plain descriptions of the problem, not Conventional
@@ -283,6 +290,28 @@ Use `good first issue` for work that is genuinely self-contained — a clear
 acceptance criterion, one or two files, no need to understand the
 opinionated-component system first.
 
+### The GitHub mirror
+
+The repo is mirrored to
+[github.com/rootiest/fish-config](https://github.com/rootiest/fish-config),
+and **the mirror carries the same labels, by the same names**. That isn't
+cosmetic: GitHub reads the same `.github/ISSUE_TEMPLATE/` files, and a
+`labels:` entry naming a label that doesn't exist on that side is silently
+dropped rather than reported. Mirroring copies files, not repository
+settings, so **a label added here must be created on the mirror too** — no
+automation does it for you.
+
+One behavioral difference to keep in mind: **GitHub has no exclusive
+labels.** Gitea enforces one-at-a-time on `Priority/`, `Reviewed/`, and
+`Status/` by swapping the old label out; on the mirror those are ordinary
+labels and nothing stops two of a group coexisting, so there the one-of rule
+holds by convention alone.
+
+Issues and pull requests belong on the canonical Gitea repo — the template
+chooser links there first, on both sides. The mirror's tracker stays open so
+that a report which lands there anyway isn't lost, not because it's a second
+supported front door.
+
 ## Commit Conventions
 
 Commit subjects follow [Conventional Commits](https://www.conventionalcommits.org/):