Add v1 to v2 migration guide

[tonybaloney]

Adds a comprehensive migration guide for users upgrading from wily v1 to v2, covering:

• Cache rebuild: v1 JSON cache is incompatible; users must run \wily clean\ and \wily build\
• Operator changes: \mccabe\ removed (replaced by \cyclomatic), new \cognitive\ operator added
• Metric reference table: Complete list of all metrics with their operator and key
• CLI changes: New --wrap, --table-style, --json\ options; Rich replaces tabulate
• Dependency changes: radon, mccabe, tabulate removed; Rust backend handles computation
• Storage format: JSON → Parquet comparison table with schema details
• Performance improvements: Parallel analysis, Rust-native parsing, columnar storage
• CI/CD guidance: What to update in pipelines
• Quick migration checklist

The guide is added as \docs/source/migration_v2.rst\ and linked as the first item in the docs table of contents.

Closes #248

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.35%. Comparing base (75669dc) to head (ab71a9c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #250   +/-   ##
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  Coverage   90.35%   90.35%           
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  Files          20       20           
  Lines        1192     1192           
  Branches      161      161           
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  Hits         1077     1077           
  Misses         87       87           
  Partials       28       28           

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