feat: add suggested solutions to untrusted site message

[Seb-MCaw]

Closes #1985

When alr publish fails due to an untrusted origin, the message now has the form

error: Could not complete the publishing assistant:
error:    Origin host 'untrusted.site' is not a trusted site:
error:    Please open an issue at https://github.com/alire-project/alire/issues/new if you think it should be added to the list.

or, if --for-private-index is specified,

error: Could not complete the publishing assistant:
error:    Origin host 'untrusted.site' is not a trusted site:
error:    This can be configured using the 'origins.git.trusted_sites' setting.

PR creation checklist

• A test is included, if required by the changes.
• doc/user-changes.md has been updated, if there are user-visible changes.
• doc/catalog-format-spec.md has been updated, if applicable.
• BREAKING.md has been updated for major changes in alr, minor/major in catalog format.

[Copilot]

Pull request overview

This PR enhances the error message shown when alr publish fails due to an untrusted origin site. The message now provides context-specific guidance: for private index publishing, it directs users to configure the origins.git.trusted_sites setting, while for community index submissions, it prompts users to open a GitHub issue if they believe the site should be trusted.

• Updated error message format to be more specific and user-friendly
• Added conditional suggested solutions based on whether --for-private-index is used
• Updated documentation to clarify why community index has restricted trusted sites

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/alire/alire-publish.adb	Implements the new error message with context-aware suggestions for untrusted sites
src/alire/alire-settings-builtins.ads	Adds clarification about SHA1 vulnerability rationale for community index restrictions
testsuite/tests/publish/check-trusted/test.py	Updates test assertions to match new error message format
testsuite/tests/publish/private-indexes/test.py	Updates test assertions and adds validation for context-specific suggestions
testsuite/tests/publish/ssh-remote-origin/test.py	Updates test assertion to match new error message format

[Seb-MCaw]

The failure looks transient, so ready for review @mosteo.