docs: upstream tracking in README + daily PR mirror (#13)

* docs: replace changelog with upstream divergence table in README

Problem: fork changes were not documented anywhere user-facing. The
auto-generated CHANGELOG.md tracked upstream releases, not fork
divergence.

Solution: delete CHANGELOG.md and add a collapsed <details> block to
README.md with two tables — cherry-picked PRs (by number, with commit
hashes) and triaged upstream issues (by number, with status and
resolution). This gives users a single place to see what the fork
offers over upstream.

* ci: add daily upstream PR mirror workflow

Problem: new upstream PRs required manual monitoring of
stevearc/oil.nvim to discover.

Solution: add a GitHub Actions workflow that runs daily at 08:00 UTC
and creates issues in the fork for any upstream PRs opened in the last
24 hours. Skips dependabot PRs and deduplicates by title prefix.
Issues are labeled upstream/pr for easy filtering.

.github/workflows/mirror_upstream_prs.yml

@@ -0,0 +1,85 @@
+name: Mirror Upstream PRs
+on:
+  schedule:
+    - cron: "0 8 * * *"
+  workflow_dispatch:
+
+permissions:
+  issues: write
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Mirror new upstream PRs
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const upstream = { owner: 'stevearc', repo: 'oil.nvim' };
+            const fork = { owner: 'barrettruth', repo: 'oil.nvim' };
+
+            const since = new Date();
+            since.setDate(since.getDate() - 1);
+            const sinceISO = since.toISOString();
+
+            const { data: prs } = await github.rest.pulls.list({
+              ...upstream,
+              state: 'open',
+              sort: 'created',
+              direction: 'desc',
+              per_page: 30,
+            });
+
+            const recentPRs = prs.filter(pr => {
+              if (new Date(pr.created_at) < since) return false;
+              if (pr.user.login === 'dependabot[bot]') return false;
+              if (pr.user.login === 'dependabot-preview[bot]') return false;
+              return true;
+            });
+
+            if (recentPRs.length === 0) {
+              console.log('No new upstream PRs in the last 24 hours.');
+              return;
+            }
+
+            const { data: existingIssues } = await github.rest.issues.listForRepo({
+              ...fork,
+              state: 'all',
+              labels: 'upstream/pr',
+              per_page: 100,
+            });
+
+            const mirroredNumbers = new Set();
+            for (const issue of existingIssues) {
+              const match = issue.title.match(/^upstream#(\d+)/);
+              if (match) mirroredNumbers.add(parseInt(match[1]));
+            }
+
+            for (const pr of recentPRs) {
+              if (mirroredNumbers.has(pr.number)) {
+                console.log(`Skipping PR #${pr.number} — already mirrored.`);
+                continue;
+              }
+
+              const labels = pr.labels.map(l => l.name).join(', ') || 'none';
+
+              await github.rest.issues.create({
+                ...fork,
+                title: `upstream#${pr.number}: ${pr.title}`,
+                body: [
+                  `Mirrored from [stevearc/oil.nvim#${pr.number}](${pr.html_url}).`,
+                  '',
+                  `**Author:** @${pr.user.login}`,
+                  `**Labels:** ${labels}`,
+                  `**Created:** ${pr.created_at}`,
+                  '',
+                  '---',
+                  '',
+                  pr.body || '*No description provided.*',
+                ].join('\n'),
+                labels: ['upstream/pr'],
+              });
+
+              console.log(`Created issue for upstream PR #${pr.number}: ${pr.title}`);
+            }