feat: rule severity levels and granular exit codes

[SpollaL]

Summary

• severity: warning | error field on rules (defaults to error). Warning rules display results but do not cause pipeline failure.
• Granular exit codes replace the old binary 0/1:

Code	Meaning
0	All rules passed
1	One or more error-severity rules failed
2	Only warning-severity rules triggered (no errors)
3	Invalid rules file or schema mismatch
4	Data file not found or unreadable

• Agents can branch on exit code without parsing stdout.
• Table output gains a SEVERITY column.

Example

rules:
  - name: id_not_null
    column: id
    check: not_null
    severity: error      # blocks pipeline

  - name: phone_format
    column: phone
    check: regex
    pattern: '^\+?[\d\s\-]{7,15}$'
    severity: warning    # alerts but doesn't block

Test plan

• Severity defaults to error when omitted from YAML
• severity: warning and severity: error parse correctly
• Warning rule with violations → Fail status + Warning severity in result
• Error rule with violations → Fail status + Error severity
• Mixed error+warning failures → exit code 1
• Warning-only failures → exit code 2
• All pass → exit code 0
• Empty result set → exit code 0

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