Move FORMAT.md to docs/ and contrib/ to bin/, fix bash script style (closes #74)

FORMAT.md moves to docs/ and every reference follows; its two relative
links to mfer/mf.proto now point up one directory, its text is otherwise
unchanged. contrib/usage.sh moves to bin/, not docs/: it is a script you
run (it builds mfer, then generates and checks a manifest of the repo),
not reading material. Both scripts now use #!/usr/bin/env bash,
set -euo pipefail and a main function. bin/gitrev.sh still exits non-zero
outside a git checkout, so the Makefile still falls back to unknown.
.gitignore now ignores only the built bin/mfer rather than all of bin/,
which holds tracked scripts.

Model: opus-5-5

bin/usage.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# usage.sh - Generate and check a manifest from the repo
+# Run from repo root: bin/usage.sh
+
+cleanup() {
+    rm -rf "$TMPDIR"
+}
+
+main() {
+    TMPDIR=$(mktemp -d)
+    MANIFEST="$TMPDIR/index.mf"
+    trap cleanup EXIT
+
+    echo "Building mfer..."
+    go build -o "$TMPDIR/mfer" ./cmd/mfer
+
+    "$TMPDIR/mfer" generate -o "$MANIFEST" .
+    "$TMPDIR/mfer" check --base . "$MANIFEST"
+}
+
+main "$@"