fix: restore alias compatibility for community extensions (#2110) (#2125)

Relax alias validation in _collect_manifest_command_names() to only
enforce the 3-part speckit.{ext}.{cmd} pattern on primary command
names. Aliases retain type and duplicate checking but are otherwise
free-form, restoring pre-#1994 behavior.

This unblocks community extensions (e.g. spec-kit-verify) that use
2-part aliases like 'speckit.verify'.

Fixes #2110