Add dev-notes convention for feature branches

Introduce docs/dev-notes/ for living design notes on unmerged feature
branches (one Markdown file per branch), so collaborators and the AI agents
they work with can pick up a branch's context from the diff. Documented in
AGENTS.md and a directory README; seeded with the opening-template-on-type
note.

Generated with the assistance of an AI coding tool.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/dev-notes/README.md

@@ -0,0 +1,28 @@
+<!-- This file was generated with the assistance of an AI coding tool. -->
+
+# Developer notes (in-progress features)
+
+This directory holds **living design/working notes for unmerged feature branches**,
+one Markdown file per feature, named after its branch (e.g.
+`opening-template-on-type.md`).
+
+## Purpose
+
+A shared scratchpad so collaborators — and the AI agents they work with — can pick up
+the context behind an in-progress branch: the problem, the design decisions and the
+*why*, dead ends already ruled out, and what still needs testing. Because the note is
+committed on the branch, it travels with the PR and shows up in the diff, so it is
+discoverable without anyone being told where to look.
+
+## How to use it (humans and agents)
+
+- **Before working on a feature branch**, read its note here if one exists.
+- **As the PR is refined**, keep the note current — append decisions, correct things
+  that changed, update the test checklist.
+- **One file per feature**, named after the branch.
+
+## Lifecycle
+
+These are *not* permanent user documentation. When a PR merges, either remove its note
+or promote the durable parts (the load-bearing "why") into code comments or the regular
+docs, so stale notes do not accumulate on the default branch.