fix(placeholders): add fast-path guards and complete test/doc coverage for apm-input-token

- Add strings.Contains("${input:") guard before regexp in ContainsBodyToken,
  MaskBodyTokens, and stripBodyTokens to avoid regex cost on non-matching text
- Add APMInputToken to TestIsKnown and TestValidate all-valid lists
- Add apm-input-token to MDS004 row in placeholder-grammar.md opt-in table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Cua8NLiPy6fhj58LvUWf1

Author: claude

docs/background/concepts/placeholder-grammar.md

@@ -75,7 +75,7 @@ from `internal/placeholders`:
 | Rule ID | Rule name                        | Useful tokens                                                             |
 | ------- | -------------------------------- | ------------------------------------------------------------------------- |
 | MDS003  | `heading-increment`              | `heading-question`, `placeholder-section`, `var-token`                    |
-| MDS004  | `first-line-heading`             | `heading-question`, `var-token`, `placeholder-section`                    |
+| MDS004  | `first-line-heading`             | `heading-question`, `var-token`, `placeholder-section`, `apm-input-token` |
 | MDS018  | `no-emphasis-as-heading`         | `var-token`, `heading-question`, `placeholder-section`, `apm-input-token` |
 | MDS020  | `required-structure`             | `cue-frontmatter`                                                         |
 | MDS023  | `paragraph-readability`          | `var-token`, `heading-question`, `placeholder-section`, `apm-input-token` |

internal/placeholders/placeholders.go

@@ -81,7 +81,7 @@ func ContainsBodyToken(text string, tokens []string) bool {
 				return true
 			}
 		case APMInputToken:
-			if apmInputRe.MatchString(text) {
+			if strings.Contains(text, "${input:") && apmInputRe.MatchString(text) {
 				return true
 			}
 		}
@@ -109,7 +109,9 @@ func MaskBodyTokens(text string, tokens []string) string {
 				return neutralText[PlaceholderSection]
 			}
 		case APMInputToken:
-			text = apmInputRe.ReplaceAllLiteralString(text, neutralText[APMInputToken])
+			if strings.Contains(text, "${input:") {
+				text = apmInputRe.ReplaceAllLiteralString(text, neutralText[APMInputToken])
+			}
 		}
 	}
 	return text
@@ -144,7 +146,9 @@ func stripBodyTokens(text string, tokens []string) string {
 				return ""
 			}
 		case APMInputToken:
-			text = apmInputRe.ReplaceAllLiteralString(text, "")
+			if strings.Contains(text, "${input:") {
+				text = apmInputRe.ReplaceAllLiteralString(text, "")
+			}
 		}
 	}
 	return text

internal/placeholders/placeholders_test.go

@@ -12,6 +12,7 @@ func TestIsKnown(t *testing.T) {
 	assert.True(t, placeholders.IsKnown(placeholders.HeadingQuestion))
 	assert.True(t, placeholders.IsKnown(placeholders.PlaceholderSection))
 	assert.True(t, placeholders.IsKnown(placeholders.CUEFrontmatter))
+	assert.True(t, placeholders.IsKnown(placeholders.APMInputToken))
 	assert.False(t, placeholders.IsKnown("unknown-token"))
 	assert.False(t, placeholders.IsKnown(""))
 }
@@ -25,6 +26,7 @@ func TestValidate(t *testing.T) {
 		placeholders.HeadingQuestion,
 		placeholders.PlaceholderSection,
 		placeholders.CUEFrontmatter,
+		placeholders.APMInputToken,
 	}))
 	err := placeholders.Validate([]string{"bad-token"})
 	assert.ErrorContains(t, err, `unknown placeholder token "bad-token"`)