feat(#998): add comment-starts-with-article lint (#1005)

Adds a new XSL-based lint that reports a warning when an object's
comment starts with an article ("a", "an", or "the"). Such articles
add no information and make documentation needlessly verbose.

The check is case-insensitive and only matches an article followed by
whitespace, so words that merely begin with the same letters (e.g.
"Another", "Theory", "Andes") are not flagged.

Includes the motive document and YAML packs covering the "a", "an" and
"the" articles, a clean comment, and the article-prefix edge cases.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: Mirletti

src/main/resources/org/eolang/lints/comments/comment-starts-with-article.xsl

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+ * SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+ * SPDX-License-Identifier: MIT
+-->
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" id="comment-starts-with-article" version="2.0">
+  <xsl:import href="/org/eolang/funcs/lineno.xsl"/>
+  <xsl:import href="/org/eolang/funcs/defect-context.xsl"/>
+  <xsl:output encoding="UTF-8" method="xml"/>
+  <xsl:template match="/">
+    <defects>
+      <xsl:for-each select="/object/comments/comment[matches(., '^(a|an|the)\s', 'i')]">
+        <xsl:element name="defect">
+          <xsl:variable name="line" select="eo:lineno(@line)"/>
+          <xsl:attribute name="line">
+            <xsl:value-of select="$line"/>
+          </xsl:attribute>
+          <xsl:if test="$line = '0'">
+            <xsl:attribute name="context">
+              <xsl:value-of select="eo:defect-context(.)"/>
+            </xsl:attribute>
+          </xsl:if>
+          <xsl:attribute name="severity">
+            <xsl:text>warning</xsl:text>
+          </xsl:attribute>
+          <xsl:text>The comment starts with an article ("a", "an", or "the"), which is redundant and should be removed</xsl:text>
+        </xsl:element>
+      </xsl:for-each>
+    </defects>
+  </xsl:template>
+</xsl:stylesheet>

src/main/resources/org/eolang/motives/comments/comment-starts-with-article.md

@@ -0,0 +1,25 @@
+# Comment Starts With Article
+
+Comments should not start with an article, such as `a`, `an`, or `the`.
+An article at the beginning of a comment adds no information and makes
+the documentation more verbose than necessary.
+
+Incorrect:
+
+```eo
+# The object that calculates the sum.
+[] > foo
+  42 > @
+```
+
+Correct:
+
+```eo
+# Object that calculates the sum.
+[] > foo
+  42 > @
+```
+
+The check is case-insensitive, so `A`, `An`, and `The` are flagged as
+well. Words that merely begin with the same letters as an article, such
+as `Another` or `Theory`, are not affected.

src/test/resources/org/eolang/lints/packs/single/comment-starts-with-article/allows-comment-without-article.yaml

@@ -0,0 +1,11 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/comments/comment-starts-with-article.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=0]
+input: |
+  # Object that calculates the sum of two numbers.
+  [] > foo
+    42 > @

src/test/resources/org/eolang/lints/packs/single/comment-starts-with-article/allows-word-starting-like-article.yaml

@@ -0,0 +1,11 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/comments/comment-starts-with-article.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=0]
+input: |
+  # Another object that wraps a theory of numbers.
+  [] > foo
+    42 > @

src/test/resources/org/eolang/lints/packs/single/comment-starts-with-article/catches-a-article.yaml

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/comments/comment-starts-with-article.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='2']
+input: |
+  # a simple object that does nothing useful.
+  [] > foo
+    42 > @

src/test/resources/org/eolang/lints/packs/single/comment-starts-with-article/catches-an-article.yaml

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/comments/comment-starts-with-article.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='2']
+input: |
+  # An object that represents a single user.
+  [] > foo
+    42 > @

src/test/resources/org/eolang/lints/packs/single/comment-starts-with-article/catches-the-article.yaml

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/comments/comment-starts-with-article.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='2']
+input: |
+  # The object that calculates the sum.
+  [] > foo
+    42 > @