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/index.css

@@ -25,7 +25,7 @@
  *      than fighting it means every primitive (dialog, select, badge, button)
  *      inherits both themes for free.
  *
- * WHY THE PAIRS MATTER. CLAUDE.md's rule is "never define a bare `--accent`",
+ * WHY THE PAIRS MATTER. AGENTS.md's rule is "never define a bare `--accent`",
  * and it was paid for with an app-wide contrast bug: shadcn uses `--accent` as
  * a hover BACKGROUND and `--accent-foreground` as the text on it, so setting
  * one without the other repaints the background and leaves the text behind.