"Fix" summarise-herg-qc

[MichaelClerx]

• Updated default experiment name to match test data
• Removed qc_vals_df which referred to a non-existent args.save_dir variable and a file not currently produced by run_herg_qc, and seemed to be unused anyway
• dropped "stage 6 criteria" based on QC.R_leftover which was causing the exception below

Traceback (most recent call last):
  File "/home/michael/dev/pipeline/venv/bin/pcpostprocess", line 8, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/michael/dev/pipeline/pcpostprocess/pcpostprocess/scripts/__main__.py", line 24, in main
    summarise_herg_export.main()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/michael/dev/pipeline/pcpostprocess/pcpostprocess/scripts/summarise_herg_export.py", line 148, in main
    attrition_df = create_attrition_table(qc_df, leak_parameters_df)
  File "/home/michael/dev/pipeline/pcpostprocess/pcpostprocess/scripts/summarise_herg_export.py", line 905, in create_attrition_table
    qc_df['QC.R_leftover'] = [R_leftover_qc.loc[well] for well in subtraction_df.well.unique()]
    ~~~~~^^^^^^^^^^^^^^^^^
  File "/home/michael/dev/pipeline/venv/lib64/python3.13/site-packages/pandas/core/frame.py", line 4322, in __setitem__
    self._set_item(key, value)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/home/michael/dev/pipeline/venv/lib64/python3.13/site-packages/pandas/core/frame.py", line 4535, in _set_item
    value, refs = self._sanitize_column(value)
                  ~~~~~~~~~~~~~~~~~~~~~^^^^^^^
  File "/home/michael/dev/pipeline/venv/lib64/python3.13/site-packages/pandas/core/frame.py", line 5288, in _sanitize_column
    com.require_length_match(value, self.index)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/home/michael/dev/pipeline/venv/lib64/python3.13/site-packages/pandas/core/common.py", line 573, in require_length_match
    raise ValueError(
    ...<4 lines>...
    )
ValueError: Length of values (26) does not match length of index (768)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.95%. Comparing base (60e55ff) to head (9cd93ed).

Files with missing lines	Patch %	Lines
pcpostprocess/scripts/summarise_herg_export.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   37.79%   37.95%   +0.16%     
==========================================
  Files          14       14              
  Lines        2114     2105       -9     
==========================================
  Hits          799      799              
+ Misses       1315     1306       -9     

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[MichaelClerx]

Am assuming this is OK and moving on.
Similar changes implemented in #118