Add socket value_source flag

[superstar54]

• value_source: link | property
• Default stays link (current behavior).
• When set to property, skip link resolution and use the property value.

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.37%. Comparing base (eaff8b5) to head (d224141).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_graph/task.py	82.00%	9 Missing ⚠️
src/node_graph/socket.py	90.16%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   90.46%   90.37%   -0.09%     
==========================================
  Files          79       79              
  Lines        7980     8126     +146     
==========================================
+ Hits         7219     7344     +125     
- Misses        761      782      +21     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.