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/backend/__init__.py

@@ -37,7 +37,7 @@ import shlex
 import sys
 from abc import ABC, abstractmethod
 from contextlib import AbstractContextManager
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any, Generic, Sequence, TypeVar
 
@@ -78,6 +78,11 @@ class BottleSpec:
     # True when launched via --headless (no TTY, no interactive prompts).
     # The git-gate host-key preflight uses this to error rather than prompt.
     headless: bool = False
+    # Forge sidecar env vars (PRD forge-native-integration, chunk 1).
+    # Passed by the orchestrator at launch time; the forge sidecar reads
+    # them to connect to Gitea. Empty for non-forge runs. The agent
+    # process itself does not receive these.
+    forge_env: dict[str, str] = field(default_factory=dict)
 
 
 @dataclass(frozen=True)