feat(mini.pairs): smart Markdown code-fence expansion on 3rd backtick

Typing a 3rd backtick after mini.pairs' `` pair now expands into a fenced
code block instead of pairing into 4 backticks: the cursor lands right
after the opener (ready for a language id), and backspacing the fence
back out removes the empty closing fence too.

Also includes an already-in-progress .gitignore expansion (AGENTS/ and
agent-generated docs/ subdirs) and a plugins.lua tabs-to-spaces reformat
that were sitting uncommitted in the working tree.

.gitignore

@@ -175,3 +175,16 @@ Thumbs.db
 .[Rr][Ee][Mm][Ee][Mm][Bb][Ee][Rr]
 
 # ──────────────────────────────────────────────────────────────
+
+#   ──────────────── Added by agents-init ──────────────────
+#   agents-init --agents
+AGENTS/
+#   ────────────────────────────────────────────────────────
+
+#   ──────────────── Added by agents-init ──────────────────
+#   agents-init --plugins
+docs/superpowers
+docs/plans
+docs/specs
+docs/devlogs
+#   ────────────────────────────────────────────────────────