docs: add DECISIONS.md, an index of what was chosen and over what

"Why is this the way it is" was answered by pointing at a directory of
five specs, which means reading five specs to find out whether the
question is even addressed. DECISIONS.md answers it in one line per
decision, grouped by area, each entry naming what the choice was made
*over* and linking into the spec that argues it.

Around forty entries. Where no spec exists — the currency model, the
baked rates, the reset — the commit message is cited as the record
instead of inventing one.

Written by reading the specs rather than from memory, which surfaced
history worth having in one place: dates were widened to accept "YYYY-MM"
rather than restructured into month plus optional day, specifically to
avoid running a migration in the browser against the user's only copy of
their financial data for an identical visible outcome.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -184,6 +184,8 @@ timing that happy-dom does not reproduce.
 - `docs/PRODUCT.md` — durable product truth: who it is for, what must never break.
 - `docs/TODO.md` — what is *still* queued, each item carrying the reasoning already
   settled. Finished work is not kept there; it is in the commit messages.
+- `docs/DECISIONS.md` — an index of every significant decision and what it was
+  chosen over, pointing into the specs for the argument.
 - `docs/specs/` — design decisions and the alternatives rejected, one document
   per piece of work.
 - `docs/plans/` — task-by-task implementation plans.