chore: remove unused dead code

[nightshift-micr]

Summary

Removes 3 unused classes and 1 unused enum discovered through dead code analysis.

Removed

• Claim (models.py) - Pydantic model with zero references across src/ and tests/
• ClaimOrigin (models.py) - StrEnum only used by the removed Claim class
• SourceMaterial (parsers.py) - dataclass with zero references
• MatchResult (taxonomy.py) - dataclass with zero references

Cleanup

• Removed unused field import from taxonomy.py

Verification

• All files pass py_compile syntax check
• ruff check passes with zero violations
• 3 files changed, 1 insertion(+), 25 deletions(-)

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This PR was generated by Nightshift - autonomous code quality bot.