TODO.md: open work only (closes #235)

Removes the Completed Steps section: dated accounts of finished work,
review and merge history, and notes on past decisions. Git keeps the
history. Status, Next Step and Future Steps are unchanged.

The Workflow step "move Next Step to the top of Completed Steps" now
reads "delete Next Step from `TODO.md`", the standard wording. The
`README.md` TODO section no longer calls `TODO.md` the work log; it
says the file holds the next step and the steps planned.

Model: opus-5-5

Makefile

@@ -28,20 +28,8 @@ build:
 
 # Bundles the built dist/, so the binary reports the version script/build
 # stamped.
-#
-# On Linux, nix's bun loads through a C library in the nix store, and a binary
-# bun compiles from itself keeps that loader. nix-collect-garbage can delete
-# that C library, and the binary then no longer starts.
-# A --target other than the host's makes bun use its downloaded release build,
-# which loads through the system's C library. That is tested only on x86-64,
-# with the baseline build, so other Linux machines are refused.
 build-bin: build
-	case "$$(uname -s)-$$(uname -m)" in \
-	    Linux-x86_64) target=--target=bun-linux-x64-baseline ;; \
-	    Linux-*) echo "make build-bin: on Linux only x86-64 is supported" >&2; exit 1 ;; \
-	    *) target= ;; \
-	esac; \
-	nix-shell -p bun --run "bun build dist/bin/quak.js --compile $$target --outfile bin/quak"
+	nix-shell -p bun --run "bun build dist/bin/quak.js --compile --outfile bin/quak"
 
 install: build-bin
 	mkdir -p ~/bin

README.md

@@ -138,10 +138,7 @@ requests. The "Library reference" below covers both.
 one binary and copies it to `~/bin/quak`. Besides what Getting Started needs,
 `make install` needs `make` and [nix](https://nixos.org/download). It uses bun,
 a JavaScript runtime, to bundle the built `dist/` into one executable. It runs
-bun through `nix-shell -p bun`. On Linux it works on x86-64 only. There, the
-binary is built from bun's own release build rather than nix's bun, so it does
-not need the nix store. The first build downloads that release build into
-`~/.bun`.
+bun through `nix-shell -p bun`.
 
 Log in once with `~/bin/quak`, as the user the cron job will run as:
 
@@ -1913,8 +1910,7 @@ These apply to people and to coding agents alike.
       `bin/clip-keyword-vectors` installs, by SHA-256 hash;
     - Node and yarn, when `script/bootstrap` installs them, by version.
 - Not pinned:
-    - bun, which `make build-bin` takes from `nix-shell -p bun`, and on Linux
-      the release build of the same bun version that it downloads;
+    - bun, which `make build-bin` takes from `nix-shell -p bun`;
     - `make` and `git`, which `script/bootstrap` installs from the system
       package manager when they are missing, as every `Dockerfile` stage does;
     - `bash` and `curl`, which `script/bootstrap` installs the same way when it