fix(linter): improve rules:if: expression evaluator

Four correctness fixes to the GitLab CI expression parser in
internal/cicontext/eval.go:

- Multi-line: \n and \r are now treated as whitespace in skipWS so
  block-scalar or folded-scalar if: values with || / && on continuation
  lines evaluate correctly instead of falling back to permissive true.
- ${VAR} curly-brace variable syntax now supported in parseValue.
- Regex flags (/pattern/i, /pattern/m, /pattern/s) are now consumed and
  translated to Go (?i)/(?m)/(?s) prefixes via applyRegexFlags.
- Variable on RHS of =~ / !~: when the right operand is $VAR, the
  variable's value is interpreted as a /regex/[flags] string via
  extractRegexFromString; non-regex values fall back to permissive true.

Adds 16 new unit tests covering all four cases and a testdata fixture
(rules_if_expr.yml) exercising multi-line, ${VAR}, and /pattern/i in a
real pipeline with context flags.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

testdata/rules_if_expr.yml

@@ -0,0 +1,51 @@
+---
+# rules_if_expr.yml
+# Exercises the rules:if: expression evaluator for:
+#   - Multi-line block-scalar expressions (|| on next line)
+#   - ${VAR} curly-brace variable syntax
+#   - Regex flags (/pattern/i case-insensitive)
+#   - Parenthesised compound expressions
+# Expected: exits 0 (lints clean with no context).
+
+stages:
+  - build
+  - deploy
+
+variables:
+  DEPLOY_ENVIRONMENTS:
+    value: "staging"
+    description: "Target deployment environment"
+
+build:
+  stage: build
+  script: make build
+  rules:
+    # Multi-line expression: || on next line
+    - if: |
+        $CI_COMMIT_BRANCH == "main" ||
+        $CI_COMMIT_BRANCH == "develop"
+      when: on_success
+    - when: never
+
+deploy-feature:
+  stage: deploy
+  script: make deploy
+  rules:
+    # ${VAR} curly-brace syntax
+    - if: '${CI_COMMIT_BRANCH} != null && ${CI_COMMIT_TAG} == null'
+      when: manual
+    - when: never
+
+release:
+  stage: deploy
+  script: make release
+  rules:
+    # Case-insensitive regex flag
+    - if: '$CI_COMMIT_BRANCH =~ /^(main|master)$/i'
+      when: on_success
+    # Parenthesised compound with multi-line
+    - if: >-
+        ($CI_COMMIT_TAG != null) &&
+        ($CI_PIPELINE_SOURCE == "push" || $CI_PIPELINE_SOURCE == "web")
+      when: on_success
+    - when: never