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>

UPGRADING.md

@@ -33,8 +33,8 @@ top-level `HANDLE` block, an init routine that cannot continue — so one mappin
 covers every exit. It is declared `AKERR_NORETURN`. If you were reading a
 consumer status out of `$?`, you were never getting it: read the stack trace,
 which carries the status at full width along with its registered name, or
-install a handler that maps your own statuses into a byte. See "Exit status" in
-[README.md](README.md).
+install a handler that maps your own statuses into a byte. See
+[docs/exit-status.md](docs/exit-status.md).
 
 No ABI break. The soname stays `libakerror.so.2` and nothing you already call
 changed shape. `akerr_exit()` is a new exported symbol, so a consumer that
@@ -108,7 +108,8 @@ Still yours to coordinate:
 * **Two threads in one context at once.** Ownership moves; it does not fork.
   Hand a context over and stop touching it — the content is written with no
   lock, so the handoff is what publishes it. See "Handing an error to another
-  thread" in README.md for the pattern, and for why the queue has to be bounded.
+  thread" in docs/thread-safety.md for the pattern, and for why the queue has
+  to be bounded.
 * **`akerr_log_method` and `akerr_handler_unhandled_error`** are read on every
   error and written by nobody but you. Set them during startup, before spawning.
 * **Renaming a status while another thread looks it up.**