docs(runbook): how to identify a deployed build, and why the version stamp lies

A 2026-08-11 investigation treated the windev production binary as having no
traceable provenance. Both pieces of evidence were normal output of our own
build and deploy procedure, and the binary was fine — but nothing in the repo
said so, so it cost a forensics pass to establish.

The version stamp is the first trap. SHA stamping landed in ec6f82b (TST-11)
and was broken on Windows until 0152180 (NEXT-09) a month later: the trailing
backslash in MSBuildThisFileDirectory escaped the Exec's closing quote, and
because the target runs ContinueOnError with ConsoleToMSBuild, git's stderr was
stamped as the revision. Every Windows build in that window reads
`0.1.2+fatal: cannot change to ...`. The point an operator needs is that this
is non-diagnostic in *both* directions — it neither incriminates a build nor
confirms one — which is not obvious from a stamp that looks like a failure.

The second is that the build directory is *supposed* to be gone: deploys build
from a detached worktree so the host's checkout stays on its own branch, and
remove it afterwards.

Records what does identify a build instead, cheapest first — including the wire
probe for the worker, since plain Write/Write2 carrying statuses[0] separates
53f69cd from b948e69 without host access or symbols — plus the deploys to date.

Also records something that went unlogged and is the reason this read as a
mystery rather than an improvement: the 2026-08-09 deploy returned the x86
worker to mainline. Production had been running dd7ca163, contained only by
origin/test/client-e2e-coverage and not an ancestor of main, so the worker in
production was not rebuildable from any mainline commit.

docs/Diagnostics.md

@@ -219,6 +219,7 @@ The order matters: putting the logging scope first ensures that authentication f
 
 ## Related Documentation
 
+- [Identifying A Deployed Build](./runbooks/IdentifyingADeployedBuild.md) — mapping a running binary back to a commit, and why the `InformationalVersion` stamp cannot be trusted on Windows builds from 2026-07-09 to 2026-08-10
 - [Sessions](./Sessions.md)
 - [gRPC](./Grpc.md)
 - [Authentication](./Authentication.md)