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/db.py

@@ -20,6 +20,7 @@ CREATE TABLE IF NOT EXISTS projects (
     pm_prompt TEXT,
     claude_md_path TEXT,
     forgejo_repo TEXT,
+    language TEXT DEFAULT 'ru',
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
 
@@ -163,10 +164,20 @@ def get_connection(db_path: Path = DB_PATH) -> sqlite3.Connection:
     return conn
 
 
+def _migrate(conn: sqlite3.Connection):
+    """Run migrations for existing databases."""
+    # Check if language column exists on projects
+    cols = {r[1] for r in conn.execute("PRAGMA table_info(projects)").fetchall()}
+    if "language" not in cols:
+        conn.execute("ALTER TABLE projects ADD COLUMN language TEXT DEFAULT 'ru'")
+        conn.commit()
+
+
 def init_db(db_path: Path = DB_PATH) -> sqlite3.Connection:
     conn = get_connection(db_path)
     conn.executescript(SCHEMA)
     conn.commit()
+    _migrate(conn)
     return conn