refactor!: Migrate runners cmd from string to argv (#134)

Migrate command execution to structured argv lists across the plugin, replacing shell-string command construction with `vim.system` argv execution. The runner API now exposes argv-based execution as the default path:

   - `runner.execute_command` -> `runner.execute`
   - `runner.execute_command_async` -> `runner.execute_async`
   - `runner.execute_command_raw` -> `runner.execute_raw`
   - `runner.execute_command_raw_async` -> `runner.execute_raw_async`
   - `runner.execute_command_sync` was removed; use `runner.execute` and handle the result directly

BREAKING CHANGES:
   - `cmd.new({ args = ... })` now requires `args` as `string[]` argv tokens, not a single string.
   - `cmd.log({ ... })` now uses `raw_flags` as `string[]` for raw passthrough flags; `raw = "..."` is no longer supported.
   - Direct users of `jj.core.runner` must migrate from the old `execute_command*` string helpers to the new argv-based `execute*` helpers.

lua/jj/core/args.lua

@@ -0,0 +1,12 @@
+local M = {}
+
+--- Build a jj fileset string literal for a path.
+--- jj path arguments use fileset syntax, so special characters like `$`
+--- must be wrapped in jj string quotes.
+---@param path string
+---@return string
+function M.fileset(path)
+	return string.format('"%s"', path:gsub("\\", "\\\\"):gsub('"', '\\"'))
+end
+
+return M