docs: rename repository from oil.nvim to canola.nvim

Problem: the fork shared the same name as upstream, making it difficult
to distinguish and discover independently.

Solution: rename the repository to canola.nvim — a type of oil, making
the lineage obvious while establishing a distinct identity. Update all
references in the README, rockspec, and issue templates.

.github/ISSUE_TEMPLATE/config.yaml

@@ -1,5 +1,5 @@
 blank_issues_enabled: false
 contact_links:
   - name: Questions
-    url: https://github.com/barrettruth/oil.nvim/discussions
+    url: https://github.com/barrettruth/canola.nvim/discussions
     about: Ask questions and discuss ideas