Fix output truncation bug, add language support for agent responses

Bug 1 — Output truncation:
  _run_claude() was replacing raw stdout with parsed sub-field which
  could be a dict (not string). run_agent() then saved dict.__repr__
  to DB instead of full JSON. Fixed: _run_claude() always returns
  string output; run_agent() ensures string before DB write.
  Added tests: full_output_saved_to_db, dict_output_saved_as_json_string.

Bug 2 — Language support:
  Added projects.language column (TEXT DEFAULT 'ru').
  Auto-migration for existing DBs (ALTER TABLE ADD COLUMN).
  context_builder passes language in project context.
  format_prompt() appends "## Language\nALWAYS respond in {language}"
  at the end of every prompt.
  CLI: kin project add --language ru (default: ru).
  Tests: language in prompt for ru/en, project creation, context.

112 tests, all passing. ~/.kin/kin.db migrated (vdol: language=ru).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/context_builder.py

@@ -109,6 +109,7 @@ def _slim_project(project: dict) -> dict:
         "name": project["name"],
         "path": project["path"],
         "tech_stack": project.get("tech_stack"),
+        "language": project.get("language", "ru"),
     }
 
 
@@ -209,4 +210,13 @@ def format_prompt(context: dict, role: str, prompt_template: str | None = None)
         sections.append(prev if isinstance(prev, str) else json.dumps(prev, ensure_ascii=False))
         sections.append("")
 
+    # Language instruction — always last so it's fresh in context
+    proj = context.get("project")
+    language = proj.get("language", "ru") if proj else "ru"
+    _LANG_NAMES = {"ru": "Russian", "en": "English", "es": "Spanish", "de": "German", "fr": "French"}
+    lang_name = _LANG_NAMES.get(language, language)
+    sections.append(f"## Language")
+    sections.append(f"ALWAYS respond in {lang_name}. All summaries, analysis, comments, and recommendations must be in {lang_name}.")
+    sections.append("")
+
     return "\n".join(sections)