Add JSON output format and a separate interpreter binary

deck-engine gains --format json (language-independent, slug-based)
so a new dist/interpreter-cli can score how significant a day's
transits are without linking the engine itself — it depends only on
the JSON shape, via its own minimal parser. Also documents the full
reading pipeline end to end (CLAUDE.md, README, docs/) and adds
docs/reading.md, a non-technical explanation of what a daily reading
contains and means.

docs/reading.md

@@ -0,0 +1,100 @@
+# Your daily reading, explained
+
+This page describes what actually happens when you get a reading —
+no code, no jargon you haven't already agreed to. If you just want to
+know "what am I looking at", start here.
+
+## The one-time setup: telling it who you are
+
+Before you can get a personal reading, you tell it a few things about
+your birth, once:
+
+- the **date** and **time** you were born, and which time zone that
+  clock time was in
+- **where** you were born (latitude/longitude)
+
+That's it. This information never changes and is only used to work out
+exactly where the planets were, and which zodiac sign was rising, at the
+moment you were born — your **natal chart**. Everything else the app
+does is compared against that one fixed reference point.
+
+## What happens every day
+
+Each day, the app does two independent things and then combines them:
+
+1. **It looks at the sky right now** and compares it to your natal
+   chart from step one — this tells you what's changing today,
+   relative to your own personal baseline (not a generic horoscope).
+2. **It shuffles a tarot deck**, using today's date as the shuffle
+   "seed". This means: if you check your reading three times on the
+   same day, you get the exact same 10 cards, in the same order, every
+   time — it only changes when the date changes. There's no randomness
+   involved beyond that; today's date fully determines today's spread.
+
+## The three parts of a reading
+
+### 1. Your natal chart
+
+A snapshot of the sky at the moment you were born: where the Sun,
+Moon, and each planet were, which zodiac sign was rising on the
+eastern horizon at that moment (your **Ascendant**), and which of the
+12 astrological "houses" each planet falls into as a result. Think of
+this as your personal baseline — it doesn't change day to day, and
+every other part of the reading is measured against it.
+
+### 2. Today's sky (your "transits")
+
+Where the Sun, Moon, and planets are positioned *today*, plus:
+
+- the current **Moon phase** (New, Waxing Crescent, Full, and so on)
+- any **aspects** — meaningful angles formed between where a planet is
+  today and where a planet sat in your natal chart. For example, "the
+  Sun is opposite your natal Moon today" is one aspect. These are the
+  traditional signals astrologers use to say a day is emotionally
+  charged, a good day for decisions, a day of tension, etc. Not every
+  day has aspects; some days are quiet.
+
+Each planet's position today is also placed into one of *your* 12
+houses (not a fresh chart for right now) — that's the standard way
+astrologers read "what house is this transit happening in for me".
+
+### 3. Your tarot spread (the Celtic Cross)
+
+A classic 10-card tarot layout, drawn from the 22 Major Arcana cards
+(The Fool, The Magician, and so on through The World). Each of the 10
+positions asks a different question about your day or situation — for
+example, "what covers you right now", "what crosses/challenges you",
+"your hopes and fears", "the likely outcome". Every card can land
+**upright** or **reversed**, which traditionally shades or inverts its
+meaning. The order and meanings of the 10 positions, and the meaning
+of each card, come from A. E. Waite's original 1911 description of the
+spread — the same source most modern tarot decks trace back to.
+
+## How "big" is today?
+
+Not every day is equally eventful astrologically. A separate,
+optional step can look at today's aspects (part 2, above) and works
+out an overall **significance level** for the day — Quiet, Notable,
+Significant, or Major — based on which planets are involved, how exact
+the angle is, and whether it touches your Sun or Moon (the two most
+personally weighted points in a chart). On a genuinely Major day, it
+also calls out the single most important thing happening, as a hint
+that it's worth paying closer attention to your tarot spread that day
+rather than treating it as routine. Quiet days still get a full
+reading — this step only adds a "pay attention today" flag, it never
+removes anything.
+
+## What a day's reading actually gives you
+
+Put together, one day's reading contains: your natal chart (for
+reference), today's sky and how it's speaking to your chart, how
+significant today is overall, and your 10-card Celtic Cross spread for
+the day. Right now these sit side by side — the astrology tells you
+*how eventful* today is and *where* to pay attention, and the tarot
+spread gives you the actual guidance to sit with. They don't yet get
+woven into a single narrative (e.g. a spread explained *in light of*
+today's significant transit) — that combined storytelling is the next
+piece to build, not something you get today.
+
+*(This page, like that piece, is still a work in progress — expect it
+to grow as the reading itself does.)*