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>

tests/err_threads_handoff.c

@@ -39,10 +39,10 @@
 #define AKERR_HANDOFF_DEPTH 32
 
 /*
- * The sizing rule the README gives, made executable. Every queued error is a
- * checked-out pool slot, and so is every producer's error in flight. Outrun the
- * pool and ENSURE_ERROR_READY exits the process from inside FAIL, with no slot
- * left to raise the failure from.
+ * The sizing rule docs/thread-safety.md gives, made executable. Every queued
+ * error is a checked-out pool slot, and so is every producer's error in flight.
+ * Outrun the pool and ENSURE_ERROR_READY exits the process from inside FAIL,
+ * with no slot left to raise the failure from.
  */
 typedef char akerr_assert_handoff_fits_pool[
     (AKERR_HANDOFF_DEPTH + AKERR_TEST_THREADS < AKERR_MAX_ARRAY_ERROR) ? 1 : -1];