Remove TODO.md; track open work in the issues (closes #76)

TODO.md is deleted and the README TODO section now only points to the
issue tracker, which is where open work and open design questions live
from now on. The section heading stays because the shared repo policy
requires it. AGENTS.md points at the tracker too.

Every open item in both lists was checked against the code and the
tracker: each was either already done or already owned by an issue,
apart from one, now filed as
#121. The 14 design questions are
all on #81,
#82,
#83 and
#79.

Model: opus-5-5

AGENTS.md

@@ -27,5 +27,6 @@ source for coding standards, formatting, linting, and workflow rules.
 - The proto definition is in `mfer/mf.proto`; generated `.pb.go` files are
   committed (required for `go get` compatibility).
 - The format specification is in `FORMAT.md`.
-- See the TODO section in `README.md` for the 1.0 implementation plan and open
-  design questions.
+- Open work, open design questions included, is tracked only in the repo's
+  issues: https://git.eeqj.de/sneak/mfer/issues. There is no `TODO.md` and no
+  TODO list in `README.md`; do not add either.