fix(install): validate packaged user skill assets

[hyperb1iss]

What this changes

The Linux installer accepts the user-facing skill inventory generated by packaging and verifies its required directory and exact file count. Older manifests that omit the field remain compatible.

Why

The package generator includes user_skill_files, but the installer's strict parser did not recognize it. Installation stopped at manifest validation before it could check or stage an otherwise valid bundle.

Verification

• Added or updated tests: 20 installer payload tests pass, including populated skills, count mismatch, missing root, zero count, null, and legacy manifests.
• Added or updated docs (not applicable; the package format already contains this field)
• just verify passes locally (including allocation contracts); just check --locked also passes.
• just deny passes (not applicable; no dependency or license changes)
• just ui-test and just ui-build pass (not applicable; no UI source changes)
• just sdk-lint, just sdk-check, and just sdk-build pass (not applicable; no SDK changes)
• just python-verify passes (not applicable; no Python changes)
• just compat-check passes (not applicable; no device database changes)
• just docs-build passes (not applicable; no documentation changes)
• cd docs && zola check passes (not applicable; no documentation changes)
• Packaging scripts were syntax-checked (not applicable; no packaging script changes). The existing release-artifact regression suite passes all five tests.
• just e2e-build passes with the normal Servo stack (not applicable; no renderer or daemon integration changes)
• just e2e-build-cpu passes (not applicable; no renderer changes)
• just e2e passes against the Servo stack (not applicable to manifest parsing)
• Tested in an Ubuntu 24.04 systemd guest as an ordinary user: the rebuilt archive installs with --no-service, and the installed CLI reports 0.5.1. Service startup and cloud journeys remain separate checks.

Notes for reviewers

The new field remains optional only for older manifests. When present, it must be a positive integer and match the packaged tree. Unknown-field rejection and the existing member, hash, path, and filesystem checks remain in force.

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[hyperb1iss]

Closing as superseded. The v0.5.1 release commit on main (89951c1) landed this same fix in a more complete form: it carries the user_skill_files field, the require_user_skills split between candidate and installed manifests, and the same three test functions by name. Rebasing this branch onto main left only duplicate tests, so there is nothing left to merge. #287 has been retargeted directly at main.

~ via nova ⚡