Move outstanding work from TODO.md into the issue tracker

Fourteen issues on source.starfort.tech/andrew/libakerror, labelled by kind and
blast radius and milestoned by what they can land in: 2.0.x for anything that
breaks no ABI, 2.1.0 for additive surface, 3.0.0 for the handler-ladder rewrite.
Everything filed carries status::grooming.

Three of the fourteen were not in this file at all. They are defects in this
library that were written down in a consumer's TODO instead: IGNORE() leaking a
context (#14), the coverage target not being namespaced when embedded (#15),
and no akerrorConfigVersion.cmake being installed (#16). libakstdlib carries a
workaround for each. That is the finding worth keeping -- a library whose
consumers record its defects in their own files cannot see them, and the
workaround gets written once per consumer.

TODO.md keeps the reasoning: why the handler ladder is a major-version change,
why a copied akerr_ErrorContext is a trap in two specific fields, why
validating more inputs lowers branch coverage by construction, why the mutation
score is a floor, and why a registered name is returned by pointer.

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

tests/MUTATION.md

@@ -149,9 +149,8 @@ Findings surfaced by mutation testing:
 * **Open:** the harness builds every mutant with the default CMake options, so a
   mutant that only breaks under concurrency is judged by a suite running without
   ThreadSanitizer. Mutating under `-DAKERR_SANITIZE=thread` would close that,
-  and needs a way to pass CMake options through to the mutant build. See
-  "Mutation testing judges concurrency mutants without a sanitizer" in
-  `TODO.md`.
+  and needs a way to pass CMake options through to the mutant build. That is
+  issue #10; `TODO.md` records why the score is a floor rather than a verdict.
 
 * **Superseded:** status names now use a private sparse registry, so the old
   public `AKERR_MAX_ERR_VALUE` ceiling and its consumer ABI mismatch no longer