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/env.py

@@ -35,6 +35,7 @@ import re
 import sys
 from dataclasses import dataclass, field
 
+from .errors import MissingEnvVarError
 from .log import die
 from .manifest import Manifest
 
@@ -136,9 +137,10 @@ def resolve_env(manifest: Manifest) -> ResolvedEnv:
             host_var = env_entry_interpolated_from(raw)
             host_value = os.environ.get(host_var, "")
             if not host_value:
-                die(
+                raise MissingEnvVarError(
+                    host_var,
                     f"env entry {name} is interpolated from ${host_var}, "
-                    f"but ${host_var} is unset or empty in the host environment."
+                    f"but ${host_var} is unset or empty in the host environment.",
                 )
             forwarded[name] = host_value
         else:  # literal