refactor(entity_detector): make multi-language extensible via i18n JSON

Move all entity-detection lexical patterns (person verbs, pronouns,
dialogue markers, project verbs, stopwords, candidate character class)
out of hardcoded module-level constants and into the entity section of
each locale's JSON in mempalace/i18n/. Adds a languages parameter to
every public function so callers union patterns across the desired
locales. The default stays ("en",), so all existing callers and tests
behave unchanged.

Also adds:
- get_entity_patterns(langs) helper in mempalace/i18n/ that merges
  patterns across requested languages, dedupes lists, unions stopwords,
  and falls back to English for unknown locales
- MempalaceConfig.entity_languages property + setter, with env var
  override (MEMPALACE_ENTITY_LANGUAGES, comma-separated)
- mempalace init --lang en,pt-br flag (persists to config.json)
- Per-language candidate_pattern so non-Latin scripts (Cyrillic,
  Devanagari, CJK) can register their own character classes instead of
  being silently dropped by the ASCII-only [A-Z][a-z]+ default
- _build_patterns LRU cache keyed by (name, languages) so multi-language
  callers don't poison each other's cache slots

Why now: the open language PRs (#760 ru, #773 hi, #778 id, #907 it) only
add CLI strings via mempalace/i18n/. PR #156 (pt-br) is the first that
needed entity_detector changes and inlined a _PTBR variant of every
constant. That doesn't scale past 2-3 languages — every text gets
checked against every language's patterns regardless of relevance, and
candidate extraction still drops accented and non-Latin names.

This PR sets the standard so future locale contributors only edit one
JSON file (no Python changes), and entity detection scales linearly
with how many languages a user actually enabled, not how many ship.

mempalace/config.py

@@ -197,6 +197,42 @@ class MempalaceConfig:
         """Mapping of hall names to keyword lists."""
         return self._file_config.get("hall_keywords", DEFAULT_HALL_KEYWORDS)
 
+    @property
+    def entity_languages(self):
+        """Languages whose entity-detection patterns should be applied.
+
+        Reads from env var ``MEMPALACE_ENTITY_LANGUAGES`` (comma-separated)
+        first, then the ``entity_languages`` field in ``config.json``,
+        defaulting to ``["en"]``.
+        """
+        env_val = os.environ.get("MEMPALACE_ENTITY_LANGUAGES") or os.environ.get(
+            "MEMPAL_ENTITY_LANGUAGES"
+        )
+        if env_val:
+            return [s.strip() for s in env_val.split(",") if s.strip()] or ["en"]
+        cfg = self._file_config.get("entity_languages")
+        if isinstance(cfg, list) and cfg:
+            return [str(s) for s in cfg]
+        return ["en"]
+
+    def set_entity_languages(self, languages):
+        """Persist the entity-detection language list to ``config.json``."""
+        normalized = [s.strip() for s in languages if s and s.strip()]
+        if not normalized:
+            normalized = ["en"]
+        self._file_config["entity_languages"] = normalized
+        self._config_dir.mkdir(parents=True, exist_ok=True)
+        try:
+            with open(self._config_file, "w", encoding="utf-8") as f:
+                json.dump(self._file_config, f, indent=2, ensure_ascii=False)
+        except OSError:
+            pass
+        try:
+            self._config_file.chmod(0o600)
+        except (OSError, NotImplementedError):
+            pass
+        return normalized
+
     @property
     def hook_silent_save(self):
         """Whether the stop hook saves directly (True) or blocks for MCP calls (False)."""