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

AGENTS.md

@@ -26,7 +26,7 @@ source for coding standards, formatting, linting, and workflow rules.
 - This is a Go library + CLI tool for generating `.mf` manifest files.
 - 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`.
+- The format specification is in `docs/FORMAT.md`.
 - 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. For this repo this overrides the