Rebuild semconvkit and verifyreadmes on changes

[MrAlias]

This updates the tool build dependencies in Makefile so local tools are rebuilt when their source files change, instead of requiring manual deletion from .tools.

[Copilot]

Pull request overview

Updates the repository Makefile tool build rules so locally-built Go tools in .tools are automatically rebuilt when their inputs change, avoiding manual cleanup when iterating on tool code.

Changes:

• Rebuild tools when internal/tools/go.sum changes (in addition to go.mod).
• Add file-based prerequisites for semconvkit and verifyreadmes so changes under their source directories trigger rebuilds.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (526b68f) to head (6fd813c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7995   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        304     304           
  Lines      23429   23429           
=====================================
  Hits       19136   19136           
  Misses      3906    3906           
  Partials     387     387           

see 5 files with indirect coverage changes

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[pellared]

I am moving to the next milestone as this is very important for a new semconv release, not mandatory for OTel Go release.