Add vault locking and a cross-platform build script

Commands that change the vault hold an exclusive lock on DIR/.lock and
readers hold a shared one, taken after the passphrase prompt. A blocked
process says it is waiting. Unix uses fcntl locks and Windows uses
LockFileEx, both released by the OS if the process dies. Plan 9 and
WebAssembly fall back to an exclusive lock file.

build.sh cross-compiles static binaries for every Go target that builds
without cgo (43 today, armv6 and armv7 for 32-bit ARM Linux) and writes
SHA256SUMS.

.gitignore

@@ -1,3 +1,4 @@
 /vault
 /vault.exe
 .claude/
+/dist