Install failure from the sysroot update reaches the interface with an empty reason #652

What happens

When the step that updates a subos sysroot after an install is refused, the install fails with an empty reason on the interface wire:

E_INTERNAL: [cmdline] failed:

The reason is printed only by log::error, so it is visible with -v and nowhere else:

[error] cmdline: sysroot update refused before metadata changed: Scrt1.o@glibc-2.44.3: installed asset registration missing
[error] [cmdline] failed:
  ✗ 1 package(s) failed

A client (mcpp) gets ErrorEvent{code: E_INTERNAL, message: "[cmdline] failed: "} and has nothing to show the user. Seen in mcpp's e2e CI (mcpp-community/mcpp#792) and reproduced with 2026.10.9.2 and 2026.10.10.2.

Where

src/core/xim/installer.cpp, run_config_hook_:

• A package without a config hook returns process_xvm_operations_(...) directly (line ~2391), and *failureMessage is never set. The caller (~3705) reports InstallPhase::Failed with that empty configFailure.
• With a config hook, the same failure is reported as "config hook failed" (~2412), which is non-empty but also drops the reason.
• process_xvm_operations_ has about fifteen return false sites (~2143–2304), each of which logs its reason and returns no text. "sysroot update refused before metadata changed: …" (~2253) is one of them.

Reproduce

1. Make an XLINGS_HOME whose active subos workspace projects assets that the home's version ledger (.xlings.json versions) does not register, e.g. link subos/ from another home that has xim:glibc installed.
2. xlings install mcpplibs:cmdline@0.0.2 -y (a source-only package, no config hook).
3. Interface: E_INTERNAL: [cmdline] failed: . CLI: [cmdline] failed: with no reason unless -v.

(The state in step 1 came from a test harness, so the refusal itself may be correct; the issue is the missing reason.)

Proposal

process_xvm_operations_ returns std::expected<void, std::string> (or takes the failureMessage out-parameter), so every refusal carries its text to InstallStatus.message. The no-config-hook path then reports the same reason as the hook path, and "config hook failed" is replaced by the actual cause. A test: an install whose sysroot update is refused emits a Failed status with a non-empty message that names the asset.