docs: add the memory convention and seed MEMORY.md

AGENTS.md now states that planned work is checkbox items in ROADMAP.md marked
open, in progress, or done, and that discoveries, gotchas, notes, and bugs go to
MEMORY.md for coordination between collaborators. MEMORY.md is seeded with what
this build-up actually cost: the Unreal read-only-home crash, Blender/Blosm
headless workarounds and the spherical projection, the tiled-TIFF DEM trap, the
reference licensing and throttling rules, and the LFS silent-blob trap.

Also stages the concurrently created Unreal project files, and stops per-project
Build/ output (FileOpenOrder logs) from entering the repository.

AGENTS.md

@@ -81,7 +81,20 @@ Always use a consistent scale test, named export presets, and repeatable import
 
 The living roadmap with per-version task checklists is in **[ROADMAP.md](ROADMAP.md)**.
 
+- Planned work is listed as checkbox items in `ROADMAP.md`, each marked **open** (`- [ ]`), **in progress** (`- [-]`), or **done** (`- [x]`). Update the marks as work changes state; a checkbox that no longer matches reality is worse than none.
+
 - Each version is a `##` heading; task groups are `###` headings with `- [ ]` steps (`[-]` in progress, `[x]` done).
 - When **all** steps in a version section are done: write a short descriptive entry to `CHANGELOG.md`, then remove the version section from `ROADMAP.md` to keep it concise.
 - Finish each milestone's release checks **before** expanding scope. Cut map size and simulation complexity before compromising the core walking/driving experience.
 - Define the release checks in `Docs/release-checks.md`. If a check fails, do not tag the release; open a patch task in `ROADMAP.md` and report the failing check in `CHANGELOG.md`.
+
+## Memory
+
+**[MEMORY.md](MEMORY.md)** is the shared working memory between sessions and collaborators. Write to it whenever you:
+
+- discover a fact about the environment, a tool, or a data source that cost time to establish;
+- hit a gotcha, trap, or surprising failure mode, and what actually caused it;
+- find or fix a bug, and how it was found so it can be found again;
+- make a decision that does not fit a `ROADMAP.md` checkbox, such as a workaround, an assumption, or a rejected approach and why.
+
+Keep entries short and dated, with the evidence: the command, the log line, or the file path. Anything that belongs in a formal document (setup steps, tool versions, licensing) goes in `Docs/` as well — memory is for coordination, not a replacement for the record. Do not let `MEMORY.md` become a second roadmap: planned work stays in `ROADMAP.md`.