build: modernize repository (#27)

* build: clean up gitignore and remove empty gitmodules

Problem: .gitignore contained 48 lines of C/shared-object boilerplate
irrelevant to a Lua Neovim plugin. .gitmodules was tracked but empty.

Solution: replace .gitignore with minimal entries covering only files
this project actually produces. Delete the vestigial .gitmodules.

* build: add editorconfig and prettierrc

Problem: no editor or formatter configuration, inconsistent with
cp.nvim and diffs.nvim conventions.

Solution: add .editorconfig (2-space Lua indent, utf-8, final newline)
and .prettierrc (prose wrap, 80 cols, single quotes, no semi) matching
the other repos.

* build: add Makefile for lint and test targets

Problem: .github/pre-commit calls `make fastlint` and .github/pre-push
calls `make lint && make test`, but no Makefile existed, so the git
hooks failed.

Solution: add Makefile with lint (stylua + selene), fastlint
(pre-commit), and test (luarocks test) targets.

* docs: add fork copyright to LICENSE

Problem: LICENSE only contained the original author's copyright notice.

Solution: add a second copyright line for the fork maintainer. MIT
requires retaining the original notice; adding a line for derivative
work is standard practice.

* ci: restructure workflows to quality/test/luarocks pattern

Problem: CI used a single tests.yml for linting, typechecking, and
testing. No conditional path filtering, no markdown format check, and
a stale mirror_upstream_prs.yml and duplicate luarocks.yml existed.

Solution: replace tests.yml with quality.yaml (stylua, selene,
lua-typecheck, prettier with dorny/paths-filter) and test.yaml
(nvim-busted, stable+nightly matrix). Update luarocks.yaml to
reference quality.yaml. Delete mirror_upstream_prs.yml and duplicate
luarocks.yml. Fix automation workflow sender check.

* build: rewrite issue templates

Problem: issue templates used upstream stevearc references, severity
dropdowns, outdated lazy.nvim bootstrap, and the .yml extension
inconsistent with other repos.

Solution: replace with .yaml templates matching cp.nvim/diffs.nvim
style. Bug report uses prerequisites checkboxes, checkhealth output,
modern lazy.nvim bootstrap with vim.g.oil pattern. Feature request
uses problem/solution/alternatives format. Add config.yaml to disable
blank issues and link discussions.

* docs: rewrite README

Problem: README contained upstream triage tables, severity dropdowns,
the old setup() pattern, a tree view question, and references to
stevearc/oil.nvim as the primary source.

Solution: full rewrite matching cp.nvim/diffs.nvim style with bold
tagline, features list, requirements, installation, documentation,
FAQ (lazy.nvim setup with vim.g.oil, migration guide, alternatives),
and acknowledgements crediting the original author.

* revert: remove Makefile

Problem: Makefile was added in b9279b5 but was previously deleted
intentionally.

Solution: remove it.

.github/workflows/quality.yaml

@@ -0,0 +1,90 @@
+name: quality
+
+on:
+  workflow_call:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  changes:
+    runs-on: ubuntu-latest
+    outputs:
+      lua: ${{ steps.changes.outputs.lua }}
+      markdown: ${{ steps.changes.outputs.markdown }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dorny/paths-filter@v3
+        id: changes
+        with:
+          filters: |
+            lua:
+              - 'lua/**'
+              - 'plugin/**'
+              - 'spec/**'
+              - '*.lua'
+              - '.luarc.json'
+              - '*.toml'
+            markdown:
+              - '*.md'
+
+  lua-format:
+    name: Lua Format Check
+    runs-on: ubuntu-latest
+    needs: changes
+    if: ${{ needs.changes.outputs.lua == 'true' }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: JohnnyMorganz/stylua-action@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version: 2.1.0
+          args: --check lua spec
+
+  lua-lint:
+    name: Lua Lint Check
+    runs-on: ubuntu-latest
+    needs: changes
+    if: ${{ needs.changes.outputs.lua == 'true' }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Lint with Selene
+        uses: NTBBloodbath/selene-action@v1.0.0
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          args: --display-style quiet .
+
+  lua-typecheck:
+    name: Lua Type Check
+    runs-on: ubuntu-latest
+    needs: changes
+    if: ${{ needs.changes.outputs.lua == 'true' }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run Lua LS Type Check
+        uses: mrcjkb/lua-typecheck-action@v0
+        with:
+          checklevel: Warning
+          directories: lua
+          configpath: .luarc.json
+
+  markdown-format:
+    name: Markdown Format Check
+    runs-on: ubuntu-latest
+    needs: changes
+    if: ${{ needs.changes.outputs.markdown == 'true' }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 8
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+      - name: Install prettier
+        run: pnpm add -g prettier@3.1.0
+      - name: Check markdown formatting with prettier
+        run: prettier --check .