docs: make AGENTS.md the only agent guide, restructured into five sections

AGENTS.md now holds the instructions and CLAUDE.md and GEMINI.md are
pointers to it. Three files of instructions become three files that
disagree; one file every agent reads cannot.

The content is reorganised rather than rewritten, into: the invariants,
the guards that fail the build, the validation this codebase deliberately
does not perform, the environment, and where to read next. Every guard
now names the test that catches it, and the two rules with no test behind
them are called out as such, because "only review catches this" is
information the reader needs.

Restructuring surfaced one rule that was missing here entirely — sw.js
must stay unhashed and be served no-cache — which had only ever been
written down in ARCHITECTURE.md.

Dropped: the `bun init` boilerplate code samples. Every rule they carried
is still stated; the hello-world test and the three frontend snippets are
not.

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

src/store/actions.ts

@@ -25,7 +25,7 @@ import type {
  * store.mutate(), which clones before applying and writes through afterwards.
  *
  * Every MONEY amount that flows through here is rounded to its currency's
- * minor unit (via roundMoney) before it lands in the draft, per CLAUDE.md's
+ * minor unit (via roundMoney) before it lands in the draft, per AGENTS.md's
  * "round before persisting" rule. FxRate.baseUnitsPerOne is deliberately
  * exempted in setFxRate below — an exchange rate is not money, and rounding
  * it to cents would be wrong in kind (see the comment there).