refactor(errors): introduce MissingEnvVarError for missing host env vars

Replace die()/RuntimeError at the point of detection with a typed
MissingEnvVarError, and catch it in the CLI dispatcher alongside
ManifestError. Library code stays free of stderr output; the CLI
renders all user-facing error lines in one place.

Sites updated: egress.egress_resolve_token_values (unset/empty token),
git_gate._provision_dynamic_key and revoke_git_gate_provisioned_keys
(missing forge_token_env), env.resolve_env (unset interpolated var).

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

bot_bottle/cli/__init__.py

@@ -7,6 +7,7 @@ from __future__ import annotations
 
 import sys
 
+from ..errors import MissingEnvVarError
 from ..log import Die, die, error
 from ..manifest import ManifestError
 from ._common import PROG
@@ -76,9 +77,10 @@ def main(argv: list[str] | None = None) -> int:
         die(f"unknown command: {command}")
     try:
         return handler(rest) or 0
+    except MissingEnvVarError as e:
+        error(str(e))
+        return 1
     except ManifestError as e:
-        # Manifest/config problems surface as a catchable exception;
-        # print the reason and exit non-zero (same UX die() used to give).
         error(str(e))
         return 1
     except Die as e: