per-user linux accounts are not optional any more

OFFICER_OS_USERS is gone. The platform behaves as it always would have with the
flag on, and there is nothing to enable.

Six conditionals, five of which were dead weight — provisionOsAccount,
deprovisionOsAccount and the create/delete paths each opened with an early
"not enabled on this server" return, and the API told the frontend whether to
render the Linux controls at all. Those go, along with the 'disabled'
DeprovisionResult stage, which nothing can produce now.

The sixth is the one with teeth. assertSecretsClosed opened with
`if (!OS_USERS_ENABLED) return`, described in its own comment as "a no-op when
the feature is off, so an existing install is unaffected until the owner opts
in". It is now unconditional: the server refuses to boot while any .env in the
project root is group- or world-readable. A member's shell reading .env and
printing JWT_SECRET was confirmed exploitable when this check was written, and a
prerequisite that only holds when somebody remembers to set a variable is not a
prerequisite.

Nothing to remove on the environment side — the flag was never in .env.example
or in the setup script.

Not typechecked: node_modules is empty in this tree and installs are frozen, so
tsgo could not run. All six files parse under `bun build --no-bundle`, and the
changes are deletions of dead branches plus one removed early return. Formatted
with prettier 3.9.6 via bunx rather than the pinned resolution, for the same
reason; its one unrelated reformat was reverted by hand.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/per-user-linux-accounts.md

@@ -279,7 +279,12 @@ Three bugs surfaced only by running it:
    also *unreachable*.
 3. **`platform/.env` was readable, and printing `JWT_SECRET` from a member's shell was confirmed.** This is
    the prerequisite above, demonstrated. It is now a boot check (`assertSecretsClosed`) that refuses to
-   start with `OFFICER_OS_USERS` on while any `.env` in the project root is group- or world-readable.
+   start while any `.env` in the project root is group- or world-readable.
+
+   That check was itself conditional on `OFFICER_OS_USERS` until 2026-08-12, which meant the guarantee was
+   opt-in. The flag is gone and the check is unconditional: a security prerequisite that only holds when
+   somebody remembers to set a variable is not a prerequisite. Per-user Linux accounts are now simply what
+   the platform does, so there is nothing to enable and nothing to forget.
 
 **`cd $HOME/..` succeeding is correct and worth being precise about.** `711` grants traversal, so `cd`
 works while `ls` does not — they can stand in the directory and see nothing in it. Beyond that, a real