Improve variable list semantics for MIP table names

[Copilot]

• Port variable list semantics (local_var for file/data, target_var for MIP lookup)
• Fix omission tracking clash in CLI (only raise on result < 0)
• Propagate new semantics to fremor varlist (non-MIP vars get empty string value)
• Fix test_cli_fremor_run_cmip7_case3: grid_label g99 → g999 (g99 is not a valid CMIP7 CV grid label)
• Fix test_make_simple_varlist_mip_table_no_match: update assertion for new semantics (non-MIP vars now included with '' value instead of filtered out)
• Fix test_fre_cmor_run_subtool_cmip7_case2 setup: use netCDF4 to copy+rename sos→sosV2 so local_var matches variable name in file
• Update _assert_data_matches/_assert_metadata_matches helpers to accept in_var_name param for case2 (input has sosV2, output has sos)

ian: also:

• add Contributors to README.md
• add SECURITY.md
• quote-consistency according to pylint
• a CMIP7 test that did not run before now runs (gppLut in CMIP7_land)
• adjust version from 0.1.2 to 0.9.0 for tagged release, set to 0.9.0post after publish workflow

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
fremorizer/cmor_finder.py	71.42%	2 Missing ⚠️
fremorizer/cli.py	66.66%	1 Missing ⚠️
fremorizer/cmor_mixer.py	90.90%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   95.19%   94.68%   -0.51%     
==========================================
  Files          10       10              
  Lines        1310     1317       +7     
==========================================
  Hits         1247     1247              
- Misses         63       70       +7     

Files with missing lines	Coverage Δ
fremorizer/_version.py	100.00% <100.00%> (ø)
fremorizer/cli.py	99.05% <66.66%> (-0.95%)	⬇️
fremorizer/cmor_mixer.py	91.66% <90.90%> (-0.37%)	⬇️
fremorizer/cmor_finder.py	95.69% <71.42%> (-4.31%)	⬇️

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