feat: support vim.g.oil configuration + remove release-please (#17)

* feat: support vim.g.oil declarative configuration

Problem: oil.nvim requires an imperative require("oil").setup(opts)
call to initialize. The Neovim ecosystem is moving toward vim.g.plugin
as a declarative config source that works without explicit setup calls.

Solution: fall back to vim.g.oil in config.setup() when no opts are
passed, and add plugin/oil.lua to auto-initialize when vim.g.oil is
set. Explicit setup(opts) calls still take precedence. Update docs
and add tests for the new resolution order.

Closes: barrettruth/oil.nvim#1

* build: remove release-please pipeline

Problem: the release-please action creates automated releases that
are not needed for this fork's workflow.

Solution: remove the release job from tests.yml and the
release-please branch exclusion from the review request workflow.

* fix(doc): improve readme phrasing

* doc: minor phrasing "improvements"

* ci: add luarocks release on tag push

Problem: there is no automated way to publish oil.nvim to luarocks
when a new version is tagged.

Solution: add a luarocks workflow that triggers on v* tag pushes,
runs the test suite via workflow_call, then publishes via
luarocks-tag-release. Add workflow_call trigger to tests.yml so it
can be reused.

doc/oil.txt

@@ -48,6 +48,27 @@ REQUIREMENTS                                                    *oil-requirement
 --------------------------------------------------------------------------------
 CONFIG                                                                *oil-config*
 
+Oil can be configured in two ways:
+
+1. The traditional `setup()` call: >lua
+    require("oil").setup({
+      -- your opts here
+    })
+<
+
+2. Declarative configuration via `vim.g.oil`, which does not require calling
+   `setup()`: >lua
+    vim.g.oil = {
+      -- your opts here
+    }
+<
+
+   When `vim.g.oil` is set, oil initializes automatically during plugin
+   loading. If `setup()` is called with explicit opts, those take precedence
+   over `vim.g.oil`.
+
+The full list of options with their defaults:
+
 >lua
     require("oil").setup({
       -- Oil will take over directory buffers (e.g. `vim .` or `:e src/`)