Add biblatex refsection support for publications to replace pub.bib/ref.bib coexistence

[Copilot]

This PR implements support for biblatex's refsection environment in the publications system, allowing users to use \nocite{*} instead of manually specifying \addpubs{...}. This addresses the coexistence issues between pub.bib and ref.bib files.

Problem

Previously, users had to:

• Manually specify each publication key with \addpubs{key1,key2,...}
• Manage potential conflicts between pub.bib and ref.bib
• Maintain publication lists manually when adding new entries

Solution

The new approach using refsection provides:

Before (traditional approach):

\begin{publications}
  \addpubs{myCiteKey,myCiteKey2,dummy:1,dummy:2}
  \printbibliography[heading=none,category=mypub,resetnumbers=true]
\end{publications}

After (new refsection approach):

\begin{refsection}[../reference/pub.bib]
  \begin{publications}
    \nocite{*}
    \printbibliography[heading=none,resetnumbers=true]
  \end{publications}
\end{refsection}

Benefits

• Automatic inclusion: \nocite{*} automatically includes all publications from pub.bib
• Clean separation: refsection cleanly separates personal publications from main references
• No key conflicts: Eliminates potential naming conflicts between bibliography files
• Easier maintenance: No need to manually update publication lists when adding new entries
• Backward compatible: Existing \addpubs approach continues to work

Changes

• Enhanced publications environment documentation in bithesis.dtx
• Updated template example in templates/graduate-thesis/misc/4_pub.tex to demonstrate the new approach
• Updated test cases to show both traditional and new methods
• Enhanced documentation explaining the new preferred workflow
• Maintained full backward compatibility

The implementation is minimal and focused, with no breaking changes to existing functionality.

Fixes #648.

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