build: add lint, compile, validate-parens, and :slow tag filter

I want a stronger maintainer-discipline baseline as I take over more of this project, so I added four targets that are common in Emacs-Lisp packages.

`make lint` runs `checkdoc`, `package-lint`, and `elisp-lint` over `org-drill.el`. It's informational for now and doesn't fail on findings, because the existing source has known docstring and style debt to clear. I'll re-tighten to a hard gate after the docstring pass is done. `make compile` byte-compiles the source with `byte-compile-error-on-warn nil`, matching the existing `build` target's leniency. `make validate-parens` is a fast structural check that surfaces the line of the offending paren without needing a full byte-compile pass.

I also added a `:slow` tag filter to the default ERT runners. `test-unit`, `test-integration`, `test-file`, and `coverage` now run with `'(not (tag :slow))`. Tests tagged `:slow` get skipped on the fast feedback path. `test-name` is left alone, since a pattern argument means the user wants those tests run whether or not they're tagged slow.

Cask gets `package-lint` and `elisp-lint` as development deps. `.gitignore` gets `*-autoloads.el` so the Cask build artifact stays out.

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