fix(database): include alembic files in package distribution

Move alembic configuration and migration files from repository root into the
Python package structure to ensure they are included in the wheel distribution.

This fix resolves the FileNotFoundError when running `agents init` in Docker
containers or any environment using the built wheel distribution.

Changes:
- Move alembic/ directory from repo root to
  src/cleveragents/infrastructure/database/migrations/
- Move alembic.ini to the same new location and update script_location setting
- Update MigrationRunner._find_alembic_ini() to search from the new canonical
  location within the package
- Update create_template_db.py to point to the new alembic.ini location
- Update documentation references to reflect new migration file locations
- Create __init__.py for migrations package

- The env.py file is imported when running tests that verify all modules can be
  imported without errors. However, context.config is only available when alembic
  is actually running migrations, not during normal module imports. This caused
  an AttributeError when the test tried to import the migrations.env module.

- Fix by using getattr() with a default value to safely access context.config,
  and guard all code that uses config with None checks. This allows the module
  to be safely imported while still functioning correctly during migrations.

Testing:
- Verified MigrationRunner can locate alembic.ini in new location
- Tested agents init succeeds in creating project with database
- Template database creation works correctly
- All migration tests should pass without changes

Alembic files now follow standard Python packaging conventions, making them
automatically included in wheel distributions without special configuration.

ISSUES CLOSED: #4180

scripts/create_template_db.py

@@ -48,7 +48,15 @@ def create_template(output_path: str = "build/.template-migrated.db") -> None:
 
     # Stamp alembic_version with the head revision so MigrationRunner
     # sees the database as fully migrated (no pending migrations).
-    alembic_ini = Path(__file__).resolve().parent.parent / "alembic.ini"
+    alembic_ini = (
+        Path(__file__).resolve().parent.parent
+        / "src"
+        / "cleveragents"
+        / "infrastructure"
+        / "database"
+        / "migrations"
+        / "alembic.ini"
+    )
     cfg = Config(str(alembic_ini))
     sd = ScriptDirectory.from_config(cfg)
     head = sd.get_current_head()