feat: expose stable Python API for programmatic bottle orchestration

Add bot_bottle/api.py with four public functions the orchestrator uses:
start_headless, resume_headless, freeze, and destroy. These let a
ProgrammaticBottleRunner call directly into bot_bottle instead of
shelling out to the CLI; call sites in lifecycle.py stay unchanged.

Key changes:
- BottleSpec gains forge_env field for forge sidecar credentials
- _launch_bottle returns (slug, exit_code) instead of int so start_headless
  can return the slug to callers
- All four API functions convert Die and non-zero exits to BottleError
- 27 new unit tests; existing tests updated for the new return type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/__init__.py

@@ -1 +1,11 @@
 """bot-bottle: Python implementation of the agent container launcher."""
+
+from .api import BottleError, destroy, freeze, resume_headless, start_headless
+
+__all__ = [
+    "BottleError",
+    "destroy",
+    "freeze",
+    "resume_headless",
+    "start_headless",
+]