diff --git a/PORTAL_BRIEF.md b/PORTAL_BRIEF.md
new file mode 100644
index 0000000..531c24f
--- /dev/null
+++ b/PORTAL_BRIEF.md
@@ -0,0 +1,40 @@
+# PORTAL — GODSIGH micro-lane · ▸ the orrery gets a door
+
+> **You are an execution agent (Opus).** Smallest lane of the wave — budget
+> ~an hour. GODSIGH is the mature sibling; make surgical edits only, match the
+> house style you find, and end with a short report (what shipped, how
+> verified, open questions).
+
+**Repo:** this one (`/Users/m3ultra/Documents/GODSIGH`, origin Gitea GODSIGH).
+Branch `portal/solargod` from `main@907094a` or later. Dev: `python3 serve.py
+8137`. **Do not touch** feed proxies, layer logic, or the registry.
+
+## P.1 The door
+
+SOLARGOD (the solar-system orrery, sibling app, port 8147) already links INTO
+GODSIGH from its Earth card. Reciprocate: a small fixed button/link in the HUD
+(near the mode toggle or HUD footer — pick what the existing CSS makes clean):
+**`🪐 SOLARGOD`** → opens `http://127.0.0.1:8147/` in a new tab
+(`rel="noopener"`).
+
+## P.2 The handshake — shared time
+
+Both apps serialize state in the URL hash, and SOLARGOD accepts an absolute
+sim-time token: `#t=@<unixMs>`. Build the href DYNAMICALLY at click time from
+GODSIGH's current clock:
+`http://127.0.0.1:8147/#t=@<ms of viewer.clock.currentTime>` — scrub GODSIGH
+three hours into the past, click through, and SOLARGOD opens at that same
+moment. (SOLARGOD clamps to its own valid range itself; don't pre-clamp.)
+
+## P.3 Verify (real input, in the browser)
+
+1. Click the button live → new tab opens SOLARGOD, planets rendered.
+2. Scrub GODSIGH off-live by ~3 h, click again → SOLARGOD's date readout shows
+   the scrubbed time (±1 min), chip reads SCRUBBED.
+3. GODSIGH itself: zero console errors, all layers still boot, hash state
+   still round-trips (paste a GODSIGH URL, view reproduces).
+4. If SOLARGOD isn't running, the tab just fails to load — that's fine, no
+   error handling needed beyond `noopener`.
+
+*Out of scope: any SOLARGOD-side change (its `t=@ms` support already exists);
+any shared component; any feed work.*