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/models.py

@@ -50,14 +50,15 @@ def create_project(
     pm_prompt: str | None = None,
     claude_md_path: str | None = None,
     forgejo_repo: str | None = None,
+    language: str = "ru",
 ) -> dict:
     """Create a new project and return it as dict."""
     conn.execute(
         """INSERT INTO projects (id, name, path, tech_stack, status, priority,
-           pm_prompt, claude_md_path, forgejo_repo)
-           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)""",
+           pm_prompt, claude_md_path, forgejo_repo, language)
+           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)""",
         (id, name, path, _json_encode(tech_stack), status, priority,
-         pm_prompt, claude_md_path, forgejo_repo),
+         pm_prompt, claude_md_path, forgejo_repo, language),
     )
     conn.commit()
     return get_project(conn, id)