Add /opt artifact support

[chmill-zz]

What this PR does / why we need it:

Adds /opt as a supported artifact destination, allowing packages to install files into layouts such as /opt//bin or /opt/bin

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #

Special notes for your reviewer:

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[Copilot]

Pull request overview

Adds /opt as a supported Linux artifact destination across Dalec’s package builders.

Changes:

• Adds the artifacts.opt specification and schema.
• Supports /opt installation, permissions, ownership, and capabilities for DEB and RPM packages.
• Adds documentation and cross-distribution installation tests.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
artifacts.go	Defines and validates Opt artifacts.
docs/spec.schema.json	Adds Opt to the generated schema.
load_test.go	Validates Opt capability configuration.
packaging/linux/deb/debroot.go	Implements DEB Opt artifact handling.
packaging/linux/deb/template_rules.go	Enables Opt permission overrides.
packaging/linux/rpm/template.go	Implements RPM Opt artifact handling.
packaging/linux/rpm/template_test.go	Tests RPM ownership and capabilities.
test/linux_target_test.go	Tests installed /opt layouts.
website/docs/artifacts.md	Documents Opt artifacts and examples.

[chmill-zz]

The three failed checks appear to be transient infrastructure failures rather than failures caused by this change:

• integration (Bionic)
• integration (Focal)
• integration (Jammy)

All three failed in the cross_platform subtest while building for ARM. apt timed out connecting to ports.ubuntu.com:80, leaving the package indexes unavailable and causing the subsequent build-dependency installation to fail with exit code 100. The other integration targets, unit tests, and lint checks passed, and these Ubuntu jobs have passed on neighboring CI runs.

Could a maintainer please rerun the failed jobs?

[cpuguy83]

Can you add a case for opt in integration/linux_target_test.go where we test other artifacts?

[invidian]

Overall LGTM, I have some questions/comments.