fix(gate): print the exact UTC stamp when the date gate fails (#3271)

## What

**Three separate commits landed a future-dated FNXC stamp this
evening**, each turning this blocking gate red on main (#3261 fixed six
across five engine files; #3270 fixes a third in core). This makes the
failure message actionable. Tooling only.

The message said *"Use the current date and a real clock time."* That
tells the author to use the value they already believed they had. It now
prints the exact stamp:

```
Current UTC stamp to use: 2026-07-31-23:39
```

Copy-paste instead of a second judgement call, computed only on a path
that has already failed.

## Why a message change rather than a rule

**The offsets are the evidence.** `00:50` against `23:34`; the engine
batch similar — consistently **1–2 hours into tomorrow**, not wrong
dates. That is the shape of a clock or timezone difference, not
carelessness, and no amount of restating the rule fixes a clock.
AGENTS.md already says to take the stamp from `date -u`; three actors
violated it in one evening anyway.

**I am one of those actors** — I have broken this rule twice today. So
this is not a complaint about anyone's diligence; it is an argument that
the instruction is doing less work than a printed value would.

## I made the same mistake inside this change

The FNXC comment documenting the fix was stamped **two minutes ahead**
of the real UTC time. Corrected from `date -u`.

**The gate did not catch it** — it scans `packages/` and not `scripts/`,
so FNXC stamps in the tooling itself are entirely unchecked. That is a
genuine scope gap and I am reporting rather than closing it: pulling
`scripts/` into scope would surface existing stamps across the tooling
and needs its own baseline pass, which does not belong in a message fix.

There is something clarifying about writing a future-dated stamp *in the
fix for future-dated stamps*, in a file the checker cannot see. It is
the same lesson this whole session kept producing — **an instrument's
blind spot is invisible in exactly the way its subject is** — and I
walked into it while holding the flashlight.

```
lint clean; gate prints the stamp on failure
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved validation feedback for future-dated entries by showing the
exact UTC timestamp format and value to use when corrections are needed.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

scripts/check-fnxc-future-dates.mjs

@@ -58,6 +58,13 @@ keep honest than two.
 const STAMP_TIME = /FNXC:[A-Za-z0-9_-]+\s+\d{4}-\d{2}-\d{2}-(\d{2}):(\d{2})/g;
 
 /** Hours 00-23, minutes 00-59. Returns the count of stamps whose clock time cannot exist. */
+/** The stamp an author should paste, in the project's `yyyy-MM-dd-HH:mm` form, in UTC. */
+function nowStamp() {
+  const d = new Date();
+  const p = (n) => String(n).padStart(2, "0");
+  return `${d.getUTCFullYear()}-${p(d.getUTCMonth() + 1)}-${p(d.getUTCDate())}-${p(d.getUTCHours())}:${p(d.getUTCMinutes())}`;
+}
+
 function impossibleClockTimes(source) {
   let bad = 0;
   STAMP_TIME.lastIndex = 0;
@@ -259,7 +266,20 @@ if (problems.length > 0) {
     + "the FNXC record — the project's why-does-this-exist trail — read out of order. An hour above 23\n"
     + "or a minute above 59 is not a real time at all.\n"
     + "Use the current date and a real clock time. If a count went DOWN, re-record the baseline in the\n"
-    + "same commit.\n",
+    + "same commit.\n"
+    /*
+    FNXC:FnxcDateGate 2026-07-31-23:39:
+    PRINT THE STAMP TO USE, do not just say "use the current date".
+
+    Three separate commits landed a future-dated stamp in one evening, each turning this blocking gate
+    red on main. The offsets were 1-2 hours, not wrong dates — the shape of a clock or timezone
+    difference rather than carelessness, and telling that author to "use the current date" is telling
+    them to use the value they thought they already had.
+
+    Printing the exact UTC stamp makes the correction copy-paste instead of a second judgement call.
+    Cheap: one `date -u`-equivalent read on a path that has already failed.
+    */
+    + `Current UTC stamp to use: ${nowStamp()}\n`,
   );
   process.exit(1);
 }