Refactor/preset name true syntax (#4)

* refactor(config): replace array preset syntax with preset_name = true

Problem: setup() mixed array entries (preset names) and hash entries
(custom providers keyed by filetype), requiring verbose
vim.tbl_deep_extend boilerplate to override presets.

Solution: unify under a single key=value model. Keys are preset names
or filetypes; true registers the preset as-is, a table deep-merges
with the matching preset (or registers a custom provider if no preset
matches), and false is a no-op. Array entries are dropped. Also adds
-f gfm to presets.github args so pandoc parses input as GFM.

* ci: format

* fix(presets): parenthesize gsub output to suppress redundant-return-value

* ci: remove superfluous things

* refactor: remove PreviewWatch* events and clean up docs

Problem: PreviewWatchStarted/PreviewWatchStopped were redundant with
the status() API, and the doc had a wrong author, stale INSTALLATION
format, and "watch mode" language left over from the watch → toggle
rename.

Solution: Remove the events and their tests. Fix the doc author,
rename INSTALLATION → SETUP to match sibling plugins, replace "watch
mode" with "auto-compile" throughout, and drop the events from EVENTS.

lua/preview/compiler.lua

@@ -188,11 +188,6 @@ function M.toggle(bufnr, name, provider, ctx_builder)
     end,
   })
 
-  vim.api.nvim_exec_autocmds('User', {
-    pattern = 'PreviewWatchStarted',
-    data = { bufnr = bufnr, provider = name },
-  })
-
   M.compile(bufnr, name, provider, ctx_builder(bufnr))
 end
 
@@ -205,11 +200,6 @@ function M.unwatch(bufnr)
   vim.api.nvim_del_autocmd(au_id)
   watching[bufnr] = nil
   log.dbg('unwatched buffer %d', bufnr)
-
-  vim.api.nvim_exec_autocmds('User', {
-    pattern = 'PreviewWatchStopped',
-    data = { bufnr = bufnr },
-  })
 end
 
 ---@param bufnr integer