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/errors.py

@@ -0,0 +1,18 @@
+"""bot-bottle application-level exception hierarchy.
+
+Exceptions here are caught by the CLI dispatcher (``cli/__init__.py``)
+and rendered as ``bot-bottle: error: …`` lines — same UX as ``die()``,
+but without coupling library code to stderr output."""
+
+from __future__ import annotations
+
+
+class MissingEnvVarError(Exception):
+    """Raised when a required host environment variable is unset or empty.
+
+    ``var_name`` is the exact name of the missing variable so callers
+    can display it without re-parsing the message string."""
+
+    def __init__(self, var_name: str, message: str) -> None:
+        self.var_name = var_name
+        super().__init__(message)