Split the README reference material into docs/

The README was 794 lines: the summary, the design rationale, the whole macro
reference, the threading contract, the build internals and the exit-status
specification in one file. It is now 178 lines -- summary, installation,
quickstart, and an index -- and the reference material lives in docs/, one file
per topic: architecture, usage, status-codes, uncaught-errors, exit-status,
thread-safety, building.

The prose moved as written. Inbound references followed it: UPGRADING.md,
TODO.md, include/akerror.tmpl.h and tests/err_threads_handoff.c now name the
docs/ file that owns the text they cite, and AGENTS.md says where new
documentation goes so the README does not grow back.

Five factual errors fixed in the moved text:

- Both NULL-pointer examples inverted their test. FAIL_ZERO_* fails when the
  expression is zero, so `(somePointer == NULL)` failed on a *valid* pointer.
  They now read `(somePointer != NULL)`.
- AKERROR_NOIGNORE, four times including the #define, is AKERR_NOIGNORE.
- FINISH_NORExbTURN is FINISH_NORETURN.
- "functiions" is "functions".
- The architecture link pointed at include/akerror.h, which is generated and
  not in the tree; it points at include/akerror.tmpl.h.

The quickstart is new text. It compiles under -Wall -Wextra -Werror and was run
through all three of its paths: handled usage error exits 0, unhandled IO error
prints a trace and exits with the status, success exits 0.

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

AGENTS.md

@@ -11,6 +11,13 @@ build is thread safe. CMake package and pkg-config templates are in `cmake/` and
 `akerror.pc.in`. Tests are one-file C programs in `tests/`; shared test helpers
 live beside them, such as `tests/err_capture.h` and `tests/err_threads.h`.
 
+Prose documentation lives in `docs/`, one file per topic — `architecture.md`,
+`usage.md`, `status-codes.md`, `uncaught-errors.md`, `exit-status.md`,
+`thread-safety.md`, `building.md`. `README.md` is deliberately short: summary,
+installation, quickstart, and an index into `docs/`. Add new documentation to
+the `docs/` file that owns the topic and link it from the README's index rather
+than growing the README back.
+
 ## Build, Test, and Development Commands
 
 Use an out-of-tree build: