feat: Support user-defined custom classification rules via config

[Pcmhacker-piro]

Description

Diff Guardian ships with 26 built-in classification rules (R01–R26) covering parameter removal, return type changes, visibility downgrades, etc. However, teams often have project-specific contracts that the built-in rules cannot express — for example, "every exported function must have a JSDoc @public tag" or "enum values must never be reordered." Adding support for user-defined custom rules in dg.config.json would let teams extend Diff Guardian without forking the codebase.

Steps to Reproduce

1. Open dg.config.json
2. There is no rules or customRules field to define additional classification logic
3. A team that wants to flag "JSDoc tag removed" or "decorator removed" as breaking has no way to do so

Expected Behavior

• dg.config.json should accept a customRules array:

  {
    "customRules": [
      {
        "id": "CUSTOM-01",
        "name": "JSDoc @public tag removed",
        "severity": "breaking",
        "match": {
          "type": "function",
          "property": "jsdocTags",
          "condition": "removed",
          "value": "public"
        }
      }
    ]
  }

• Custom rules should be evaluated after built-in rules
• Custom rule violations should appear in the terminal report and PR comment with a [CUSTOM-01] prefix
• npx dg rules should list both built-in and custom rules

Implementation Hints

Config Schema (src/config/):

• Extend the config type with customRules?: CustomRule[]
• Define the CustomRule interface with id, name, severity, and match conditions

Classifier Engine (src/classifier/):

• After running built-in rules, iterate over customRules and evaluate each match condition against the diff
• Map the match.condition to an AST diff predicate (e.g., removed checks if the property existed in base but not head)

Reporter (src/reporters/):

• Prefix custom rule IDs in the output so they are visually distinct from built-in rules

Affected Files

• src/config/schema.ts
• src/classifier/engine.ts
• src/reporters/terminalReporter.ts
• src/reporters/githubReporter.ts
• dg.config.json (schema update)

Labels

type:feature, level:advanced

[Pcmhacker-piro]

Hi @Aryan0628 ,
I would like to work on this issue. I have analyzed the requirements and would like to be assigned to it. Thank you!