zen CLI security review fixes (#974)

Security review follow-ups to the `zen` CLI. Each fix stands on its own commit. Grouped by category below.

## Credentials and secrets

- **Per-install random auth encryption key instead of a hardcoded literal.** The default AES key and IV used to encrypt persisted OIDC refresh tokens / OAuth client secrets were ASCII literals compiled into the public source. Replaced with 32+16 random bytes persisted to `<system-root>/auth/machinekey.dat`. `SecureRandomBytes` added in zencore/crypto wrapping BCryptGenRandom / OpenSSL / mbedTLS CTR_DRBG. Partial override (only one of `--encryption-aes-key`/`--encryption-aes-iv`) is now rejected instead of silently using the hardcoded half.
- **Wrap the machine key with OS-protected storage.** `machinekey.dat` is now a tagged format (4-byte magic + flags + wrapped-or-raw payload). Windows wraps via DPAPI (`CryptProtectData` at per-user scope) so a stolen disk copy cannot decrypt without the OS master key. macOS uses Keychain Services (GenericPassword under `org.unrealengine.zen.auth`, `kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly`). Linux uses libsecret (opt-in via `--zenlibsecret=yes`, off by default because headless servers typically have no Secret Service daemon). All platforms fall back to raw persistence with `0600` perms on POSIX when wrapping is unavailable. Legacy files from the prior commit are detected by size and still read.

> Note: argv-redaction before Sentry on crash was previously part of this PR but was superseded by `ScrubSensitiveValues()` from #989; this PR now just calls that helper instead of walking argv itself.

## Path traversal

- **Reject unsafe filenames from the remote oplog in `oplog-mirror`.** The filename from each oplog entry was joined to the mirror root without normalisation; a compromised remote could use drive letters, UNC shares, device path prefixes, absolute paths, or `..` components to write anywhere the zen user could write. An `UnsafeFileNameReason` check runs immediately after extraction, logs the offending filename, and aborts the mirror.
- **Use the resolved absolute download-spec path in `builds download`.** `--download-spec-path` was computed into a sanitised absolute path, then the original unresolved path was passed to `ParseBuildManifest`, bypassing the `MakeSafeAbsolutePath` mitigations and reading from the process cwd rather than `--local-path`.

## Input validation

- **Stop asserting on malformed `--build-id` / `--build-part-id`.** `Oid::FromHexString` asserts on bad input and `ZEN_ASSERT` is active in release, so a too-short or non-hex user value aborted the process instead of surfacing an `OptionParseException`. Routed all callers through `TryFromHexString`. Also fixes `ParseBuildPartId` reporting errors under the wrong option name.
- **Check the JSON parse error in `oplog-export --builds-metadata-path`.** The single-arg `LoadCompactBinaryFromJson` overload discarded the parser error; malformed JSON shipped a truncated compact-binary `metadata` field to the server with no indication. Switched to the two-arg overload and throws a descriptive error naming the file and reason.
- **Format the actual value in the malformed `--url` error.** The message was constructed with a literal `{}` placeholder and no `fmt::format` call, so users saw the placeholder instead of the offending URL.
- **Require `--output-path` in `cache get` unless `--as-text` is set.** Previously an empty path auto-filled from the value key / attachment hash and wrote into the process cwd; the `--as-text && empty path` stdout branch was unreachable because the auto-fill ran first.
- **Clear the cxxopts `allow_unrecognised_options` flag after permissive parse.** `ParseOptionsPermissive` set the flag on the Options it received and never cleared it, priming that Options for silent typo acceptance on any later reuse. Added `disallow_unrecognised_options()` to the vendored cxxopts (local patch — flagged at the declaration) and wrapped the toggle in RAII.

## Resource lifecycle

- **Restore signal handlers via RAII.** `wipe`, `builds`, and `oplog-mirror` installed SIGINT/SIGBREAK handlers with raw `signal()` and never restored them; an option-parse throw left the handler targeting an abort flag nothing reads. Added `zen::ScopedSignalHandler` in zen.h and applied at all three sites (builds uses `std::optional` members so the guards survive past `OnParentOptionsParsed` into the subcommand's Run).
- **Route SIGINT in `oplog-mirror` to the worker-pool abort flag.** The command declared a local `std::atomic<bool> AbortFlag` but no handler targeted it — Ctrl-C killed the process instead of cleanly aborting. Added a `MirrorAbortFlag` / `MirrorSignalCallbackHandler` pair in projectstore_impl and bound the local as a reference; existing `.store`/`.load`/capture sites unchanged.
- **Clean up the `cache get` temp download on every exit path.** `Http.Download` parks the payload in the system temp dir; a failed `MoveToFile` (cross-volume, denied target) or an exception could leave the temp file behind. The downloaded buffer is already flagged delete-on-close by `HttpClient`, so the fix is just to clear that flag after a successful `MoveToFile` so the renamed-out file isn't reaped.

## Other

- **Fix wrong URL fields in `oplog-export` / `oplog-import` builds-branch descriptions.** Two operator-facing "[builds] URL/namespace/bucket/buildsid" messages formatted `m_CloudUrl` instead of `m_BuildsUrl` / `m_BuildsHost` (copy-paste from neighbouring `[cloud]` branches), shown as empty or stale at the start of an export/import.
- **Fix "Can't find oplog in project '{}'" formatting and a "Failed top mirror" typo in projectstore_cmd.**
- **Fix a misleading `oplog-export` comment on the `--zen` scheme default** ("Assume https" vs. the `http://` the code writes).
- **Fail `ScrambleDir` when `RemoveFile` doesn't delete.** The `zen builds test` scramble phase used `(void)RemoveFile(FilePath)`, discarding both the bool return and the error. A quiet delete failure let verification run against stale state; switched to the two-arg overload and throw on false return or non-empty `error_code`.

0 files changed, 0 insertions, 0 deletions