core: prefix internal helpers with an underscore so the public rules are clear

[jbpros]

Closes #27.

Eight rules keep their names, because the docs tell people to call or define them: report, report_with_params, violations, operators, op_passed, value_at, arg and leaf_passed. The other 227 now start with _.

This breaks any policy that calls a helper directly, like ergo.row_cause.

How the rename was done

OPA lets a local variable or parameter share a rule's name, so a plain find-and-replace wasn't safe. A script renamed identifiers outside strings and field reads (.x), and would have skipped any rule body that declared a local with the same name. It found none. Calls from other packages were renamed only where written as ergo.x.

Other changes

• Four tests in ergo_test.rego call _is_violation, _definition_field and _rfc3339_shaped directly. Two pin that violations reads only the requirements, and two test the RFC 3339 gate on its own (compare_time accepts numbers through another branch), so neither can be checked through the report. They stay where they are: Regal's leaked-internal-reference doesn't apply to _test.rego files, and moving them into package ergo would put test rules into data.ergo.
• REFERENCE.md now names exactly the rules a custom operator may call, says what each does, and says the rest can change. It also drops check_passed from the list of rules that cause a loop, since it's private now, and fixes "Two rules:" over a list of three.

Checks

• opa check --strict . --ignore .github, opa fmt --list . (prints nothing), opa test . --ignore .github (642/642, the same count as main), and the --strict-builtin-errors run prints true.
• The coverage command lists no unreached lines.
• Only ergo_test.rego calls a _ rule from outside package ergo.
• The baking example's report is byte-identical to main's.
• regal lint --disable-all --enable leaked-internal-reference --ignore-files '.github/**' . finds nothing.
• 1,000 random policies and inputs give byte-identical reports and violations on main and this branch.

#34 is the only open PR touching ergo.rego, so it will need its helper calls renamed when rebased.

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