fix(linter): support bare true/false and integer literals in rules:if:

GitLab CI expressions allow unquoted true, false, and integers as
comparison operands (all treated as their string representations):

  $GATEWAY_ENABLED == true    (equivalent to == "true")
  $FEATURE_FLAG == false      (equivalent to == "false")
  $PARALLEL == 4              (equivalent to == "4")
  $ENABLED == 1 / == 0

Previously these fell through to permissive true because parseValue
only recognised $VAR, "${VAR}", quoted strings, and null. Added:
  - true/false keyword branch → returns "true"/"false"
  - integer literal branch (digits only) → returns decimal string

All three new forms are correctly excluded from longer identifier
prefixes (identByte boundary check). Adds 8 new unit tests.

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

CHANGELOG.md

@@ -9,11 +9,13 @@ This project uses [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- **`rules:if:` expression evaluator improvements** — four correctness fixes to the GitLab CI expression parser:
+- **`rules:if:` expression evaluator improvements** — six correctness fixes to the GitLab CI expression parser:
   - **Multi-line expressions** — newlines (`\n`, `\r`) are now treated as whitespace between tokens, so block-scalar `if:` values (e.g. `if: | ...`) and folded YAML scalars with `||`/`&&` on a continuation line are parsed correctly instead of falling back to permissive `true`.
   - **`${VAR}` curly-brace variable syntax** — `${CI_COMMIT_BRANCH}` is now equivalent to `$CI_COMMIT_BRANCH` everywhere a value is expected.
   - **Regex flags** — `/pattern/i`, `/pattern/m`, `/pattern/s` are now honoured; the `i` flag (case-insensitive) is translated to Go's `(?i)` prefix before compiling. Unknown flags are silently ignored.
   - **Variable as regex RHS** — `$BRANCH =~ $PATTERN` where `$PATTERN` holds a `/regex/[flags]` string is now evaluated by extracting and compiling the pattern from the variable's value; if the value is empty or does not look like a regex literal the expression falls back to permissive `true`.
+  - **`true` / `false` keywords** — bare `true` and `false` (without quotes) are now recognised as the string values `"true"` and `"false"`, matching GitLab CI's own behaviour. `$GATEWAY_ENABLED == true` and `$FEATURE_FLAG == false` now evaluate correctly.
+  - **Integer literals** — bare integers (e.g. `$PARALLEL == 4`, `$ENABLED == 1`, `$DISABLED == 0`) are now parsed as their decimal string representations and compared accordingly.
 
 - **File and line numbers on findings** — every finding now includes the source file and line where the job is defined, e.g. `[ERROR] job "deploy" (src/deploy.yml:14): …`. For jobs that come from local or fetched includes the file reflects the include source. Pipeline-level findings (workflow rules, missing stages) reference the root pipeline file.