docs: make AGENTS.md the canonical agent guide for Codex/Astra (#1742)

Codex reads AGENTS.md by default, not CLAUDE.md. Move project guidance to
AGENTS.md (trim/nest long validation notes under docs/AGENTS.md), leave
CLAUDE.md as a thin @AGENTS.md wrapper for Claude Code, and document the
Codex project_doc_max_bytes raise needed for the ~35KiB root file.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

CHANGELOG.md

@@ -25,6 +25,8 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New Features
 
+- **Codex and Astra read project guidance from `AGENTS.md`.** The canonical agent guide now lives in `AGENTS.md` (with a nested `docs/AGENTS.md` for long validation notes); `CLAUDE.md` is a thin `@AGENTS.md` wrapper for Claude Code. Codex/Astra no longer miss the old CLAUDE-only instructions.
+
 - **A busy screen's picture is laid out by the parts of the screen.** A screen is a set of handlers with no order between them, so on a hub screen the old rows-by-distance collapsed into one enormous row — the main screen of one app put 89 boxes side by side on a canvas over 28,000px wide, every line a near-horizontal sweep across all of it. The Steps tab now groups a screen's picture by region — the component that owns each handler, named in a small caption over its boxes — with each region a column where a step sits above what it sets in motion, tiled in the screen's own source order. At rest the picture hides only two things: the screen's own fan-out — one line into each region stands in for it — and lines that point back up; every other line draws where it leads, between two regions included, and selecting a step brings out its whole story in the side panel, link by link. A box nothing points at is the screen's own doing — run on render or mount, or from a binding written inline — the key says so, and selecting it lights its line from the screen with what fires it. The same app's widest screen now lays out under 3,500px with every line local, and the whole picture fits on screen when it opens. Endpoints, handlers and the in-order reading are untouched, and nothing needs a re-index: the regions come from the same walk that draws the steps.
 
 - **Where the code chooses, the picture says so once.** A helper that ends `return (await hasSeenWelcome(id)) ? '/home/' : '/welcome/'` sends the app to one of two screens, but the Steps picture drew that as two separate arrows, each carrying the whole condition with one of them negated and both cut off at the same forty characters — and before you clicked anything, neither arrow was labelled at all, so nothing said it was a choice. Now sibling arrows out of one box that are the arms of one `if`, `switch` or ternary are drawn as the choice they are: the condition is written once under the box that decides it, and each arrow out says only which way it is — `yes`, `no`, or a case's own value. They are the only arrows labelled before you select anything, so the picture reads at a glance without becoming a wall of text. A one-sided guard — an early exit, an `if` with only one side drawn — still carries its condition on the arrow, and an arrow that is reached whether or not the condition holds never claims a side. Nothing needs a re-index: the decision is read from the source at request time.