Address Copilot review feedback

- Fix regex to actually match std::to_string() by using alternation
  (the : in the lookbehind was preventing matches)
- Update error message to mention both std::to_string() and
  unqualified to_string() forms
- Correct buffer sizes for signed integer types:
  - int8_t: 5 chars (not 4) for "-128\0"
  - int16_t: 7 chars (not 6) for "-32768\0"
  - int32_t: 12 chars (not 11) for "-2147483648\0"

esphome/components/api/homeassistant_service.h

@@ -25,7 +25,9 @@ template<typename... X> class TemplatableStringValue : public TemplatableValue<s
 
  private:
   // Helper to convert value to string - handles the case where value is already a string
-  template<typename T> static std::string value_to_string(T &&val) { return to_string(std::forward<T>(val)); }
+  template<typename T> static std::string value_to_string(T &&val) {
+    return to_string(std::forward<T>(val));  // NOLINT
+  }
 
   // Overloads for string types - needed because std::to_string doesn't support them
   static std::string value_to_string(char *val) {

esphome/components/esp32_ble/ble_uuid.h

@@ -48,7 +48,7 @@ class ESPBTUUID {
 
   // Remove before 2026.8.0
   ESPDEPRECATED("Use to_str() instead. Removed in 2026.8.0", "2026.2.0")
-  std::string to_string() const;
+  std::string to_string() const;  // NOLINT
   const char *to_str(std::span<char, UUID_STR_LEN> output) const;
 
  protected:

esphome/components/voice_assistant/voice_assistant.h

@@ -83,7 +83,7 @@ struct Timer {
   }
   // Remove before 2026.8.0
   ESPDEPRECATED("Use to_str() instead. Removed in 2026.8.0", "2026.2.0")
-  std::string to_string() const {
+  std::string to_string() const {  // NOLINT
     char buffer[TO_STR_BUFFER_SIZE];
     return this->to_str(buffer);
   }

script/ci-custom.py

@@ -756,6 +756,53 @@ def lint_no_sprintf(fname, match):
     )
 
 
+@lint_re_check(
+    # Match std::to_string() or unqualified to_string() calls
+    # The esphome namespace has "using std::to_string;" so unqualified calls resolve to std::to_string
+    # Use negative lookbehind for unqualified calls to avoid matching:
+    #   - Function definitions: "const char *to_string(" or "std::string to_string("
+    #   - Method definitions: "Class::to_string("
+    #   - Method calls: ".to_string(" or "->to_string("
+    #   - Other identifiers: "_to_string("
+    # Also explicitly match std::to_string since : is in the lookbehind
+    r"(?:(?<![*&.\w>:])to_string|std\s*::\s*to_string)\s*\(" + CPP_RE_EOL,
+    include=cpp_include,
+    exclude=[
+        # Vendored library
+        "esphome/components/http_request/httplib.h",
+        # Deprecated helpers that return std::string
+        "esphome/core/helpers.cpp",
+        # The using declaration itself
+        "esphome/core/helpers.h",
+        # Test fixtures - not production embedded code
+        "tests/integration/fixtures/*",
+    ],
+)
+def lint_no_std_to_string(fname, match):
+    return (
+        f"{highlight('std::to_string()')} (including unqualified {highlight('to_string()')}) "
+        f"allocates heap memory. On long-running embedded devices, repeated heap allocations "
+        f"fragment memory over time.\n"
+        f"Please use {highlight('snprintf()')} with a stack buffer instead.\n"
+        f"\n"
+        f"Buffer sizes and format specifiers (sizes include sign and null terminator):\n"
+        f"  uint8_t:          4 chars   - %u (or PRIu8)\n"
+        f"  int8_t:           5 chars   - %d (or PRId8)\n"
+        f"  uint16_t:         6 chars   - %u (or PRIu16)\n"
+        f"  int16_t:          7 chars   - %d (or PRId16)\n"
+        f"  uint32_t:         11 chars  - %" + "PRIu32\n"
+        "  int32_t:          12 chars  - %" + "PRId32\n"
+        "  uint64_t:         21 chars  - %" + "PRIu64\n"
+        "  int64_t:          21 chars  - %" + "PRId64\n"
+        f"  float/double:     24 chars  - %.8g (15 digits + sign + decimal + e+XXX)\n"
+        f"                    317 chars - %f (for DBL_MAX: 309 int digits + decimal + 6 frac + sign)\n"
+        f"\n"
+        f"For sensor values, use value_accuracy_to_buf() from helpers.h.\n"
+        f'Example: char buf[11]; snprintf(buf, sizeof(buf), "%" PRIu32, value);\n'
+        f"(If strictly necessary, add `{highlight('// NOLINT')}` to the end of the line)"
+    )
+
+
 @lint_content_find_check(
     "ESP_LOG",
     include=["*.h", "*.tcc"],