build: replace luacheck with selene, add nix devshell and pre-commit (#20)

* build: replace luacheck with selene

Problem: luacheck is unmaintained (last release 2018) and required
suppressing four warning classes to avoid false positives. It also
lacks first-class vim/neovim awareness.

Solution: switch to selene with std='vim' for vim-aware linting.
Replace the luacheck CI job with selene, update the Makefile lint
target, and delete .luacheckrc.

* build: add nix devshell and pre-commit hooks

Problem: oil.nvim had no reproducible dev environment. The .envrc
set up a Python venv for the now-removed docgen pipeline, and there
were no pre-commit hooks for local formatting checks.

Solution: add flake.nix with stylua, selene, and prettier in the
devshell. Replace the stale Python .envrc with 'use flake'. Add
.pre-commit-config.yaml with stylua and prettier hooks matching
other plugins in the repo collection.

* fix: format with stylua

* build(selene): configure lints and add inline suppressions

Problem: selene fails on 5 errors and 3 warnings from upstream code
patterns that are intentional (mixed tables in config API, unused
callback parameters, identical if branches for readability).

Solution: globally allow mixed_table and unused_variable (high volume,
inherent to the codebase design). Add inline selene:allow directives
for the 8 remaining issues: if_same_then_else (4), mismatched_arg_count
(1), empty_if (2), global_usage (1). Remove .envrc from tracking.

* build: switch typecheck action to mrcjkb/lua-typecheck-action

Problem: oil.nvim used stevearc/nvim-typecheck-action, which required
cloning the action repo locally for the Makefile lint target. All
other plugins in the collection use mrcjkb/lua-typecheck-action.

Solution: swap to mrcjkb/lua-typecheck-action@v0 for consistency.
Remove the nvim-typecheck-action git clone from the Makefile and
.gitignore. Drop LuaLS from the local lint target since it requires
a full language server install — CI handles it.

.github/workflows/tests.yml

@@ -10,21 +10,15 @@ on:
       - main
 
 jobs:
-  luacheck:
-    name: Luacheck
+  selene:
+    name: Selene
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
-
-      - name: Prepare
-        run: |
-          sudo apt-get update
-          sudo add-apt-repository universe
-          sudo apt install luarocks -y
-          sudo luarocks install luacheck
-
-      - name: Run Luacheck
-        run: luacheck lua tests
+      - uses: NTBBloodbath/selene-action@v1.0.0
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          args: --display-style quiet .
 
   stylua:
     name: StyLua
@@ -43,9 +37,11 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
-      - uses: stevearc/nvim-typecheck-action@v2
+      - uses: mrcjkb/lua-typecheck-action@v0
         with:
-          path: lua
+          checklevel: Warning
+          directories: lua
+          configpath: .luarc.json
 
   run_tests:
     strategy: