Only set start_id on first run of a session

[rowanseymour]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 62.21%. Comparing base (c4bf518) to head (5d84f13).

Files with missing lines	Patch %	Lines
core/models/sessions.go	87.50%	1 Missing ⚠️
core/runner/runner.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   62.23%   62.21%   -0.02%     
==========================================
  Files         195      195              
  Lines       13806    13797       -9     
==========================================
- Hits         8592     8584       -8     
+ Misses       4303     4302       -1     
  Partials      911      911              

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