fix: resolve README disk_usage constraint failures (CEL cross-type numerics + YAML regex escaping)

[Copilot]

Two bugs in the README disk_usage example caused constraint compilation/evaluation errors for anyone following the quickstart.

Changes

• pkg/common/constraints.go — Add cel.CrossTypeNumericComparisons(true) to the CEL env options. number-typed params are bound as double, so comparing them against integer literals (e.g. max_depth >= 1) failed without this option.

• README.md — Fix YAML quoting on the regex constraint. The double-quoted YAML string "directory.matches('^[\\w\\s./\\-_]+$')" has its backslashes consumed by the YAML parser (\\w → \w), leaving invalid escape sequences in the CEL string literal. Changed to YAML single-quoted form which passes backslashes through verbatim:

  # Before (broken: YAML strips one layer of backslashes)
  - "directory.matches('^[\\w\\s./\\-_]+$')"
  
  # After (YAML single-quotes are literal; CEL then resolves \\w → \w for the regex)
  - 'directory.matches("^[\\w\\s./\\-_]+$")'

• pkg/common/constraints_test.go — Added test cases covering pass/fail for double params compared against integer literals (max_depth >= 1 && max_depth <= 3).