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/git_gate.py

@@ -82,6 +82,7 @@ class GitGatePlan:
 
 
 
+
 class GitGate(ABC):
     """The per-agent git-gate. Encapsulates the host-side prepare
     (upstream lift + entrypoint/hook render); the sidecar's