docs(upstream): collapse issue sections into single table (#91)

Problem: issues were split across four separate sections by status,
requiring moves between sections on every status change.

Solution: merge all issues into one `## Issues` table sorted by number
with an inline status column. Update digest script heading and row
format to match.

.github/scripts/upstream_digest.py

@@ -11,7 +11,7 @@ UPSTREAM = "stevearc/oil.nvim"
 UPSTREAM_MD = "doc/upstream.md"
 
 PRS_HEADING = "## Upstream PRs"
-ISSUES_HEADING = "## Issues — open"
+ISSUES_HEADING = "## Issues"
 
 
 def get_last_tracked_number():
@@ -128,7 +128,7 @@ def main():
     issue_rows = []
     for issue in open_issues:
         issue_rows.append(
-            f"| [#{issue['number']}]({issue['url']}) | {issue['title']} |"
+            f"| [#{issue['number']}]({issue['url']}) | {issue['title']} | open |"
         )
 
     if pr_rows: