Stamp Docker image version from the host; flush profiles on error exit (closes #75)

Docker images reported commit unknown because the build ran git inside the container while .dockerignore excludes .git, and VERSION was never overridden. script/docker and script/cibuild now compute version, commit and date on the host and pass them as build args; the Dockerfile runs no git and falls back to dev and unknown, never empty, on a bare docker build.

Profiling a failing command gave a truncated or missing profile: Entry and each command goroutine called os.Exit(1), skipping the deferred profile writers in main. Entry now returns a status that main exits with after its defers run, and command goroutines report failure through one RunOperation helper, which also restores PID-lock release and graceful shutdown on failure.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

Co-authored-by: clawbot <clawbot@noreply.example.org>

cmd/vaultik/lintdocker_test.go

@@ -304,10 +304,14 @@ func instructionText(contents string) string {
 }
 
 // indexOf returns the position of the first instruction equal to, or
-// beginning with, want; -1 if there is none.
+// beginning with, want; -1 if there is none. An `ARG NAME=default`
+// counts as beginning with `ARG NAME`, so a declared arg is found
+// whether or not it carries a default.
 func indexOf(found []string, want string) int {
 	for i, instruction := range found {
-		if instruction == want || strings.HasPrefix(instruction, want+" ") {
+		if instruction == want ||
+			strings.HasPrefix(instruction, want+" ") ||
+			strings.HasPrefix(instruction, want+"=") {
 			return i
 		}
 	}