setup: drop the agent -> oneof-variant identity map

Review feedback on #267: `_AGENT_MODEL_CONFIG_VARIANT` mapped each agent to its
`AgentModelConfig` oneof key, but the proto's field names are ucode's tool names
verbatim — claude, codex, opencode, pi, gemini, copilot — so every entry mapped a
name to itself. One use site, so the tool now serves as the key directly.

The dict's only other effect was a KeyError on an unknown agent, which was already
unreachable: `serialize_managed_config` filters to `tool in AGENT_TOOL_TO_ENUM`
before calling this, and the `AGENT_TOOL_TO_ENUM[tool]` lookup two lines down would
raise first anyway.

No new test. The variant keys are already covered — hard-coding the wrong one fails
`test_codex_model_config_has_no_model_list` and
`test_flat_list_agents_use_repeated_models`, and the round-trip through
`normalize_managed_config` asserts the alignment for every agent.

The other half of that review comment — validating a model against its agent's
dialect, so a manifest can't pin a GPT id for Claude Code — is deliberately not
here. It turned out to need a decision rather than a patch: the agent -> families
mapping already exists twice (`agents._TOOL_DISCOVERY_SOURCES` and
`managed_setup._AGENT_MODEL_FAMILIES`) and the two disagree for codex, copilot,
opencode, and pi. Picking one requires checking each agent's own writer, and the
likely outcome is that this module's opencode entry is wrong — it lists `codex`
while `build_opencode_base_urls` serves no OpenAI route — which would be a picker
bug in the wizard, not a refactor. Landing that separately.

Co-authored-by: Isaac

Author: tt-le

src/ucode/managed_setup.py

@@ -48,17 +48,6 @@
 AGENT_TOOL_TO_ENUM: dict[str, str] = {tool: enum for enum, tool in AGENT_ENUM_TO_TOOL.items()}
 MCP_TAG_TO_TYPE_ENUM: dict[str, str] = {tag: enum for enum, tag in MCP_TYPE_ENUM_TO_TAG.items()}
 
-# `AgentModelConfig` oneof variant key per agent. The server rejects a config whose variant doesn't
-# match its agent (`validateAgentModelConfig`), so this mapping is not cosmetic.
-_AGENT_MODEL_CONFIG_VARIANT: dict[str, str] = {
-    "claude": "claude",
-    "codex": "codex",
-    "opencode": "opencode",
-    "pi": "pi",
-    "gemini": "gemini",
-    "copilot": "copilot",
-}
-
 # Agents whose model config carries a flat `models` list. Claude instead uses per-family slots
 # (`ClaudeDefaultModels`), and Codex has no model list at all — it selects exactly one model.
 _FLAT_MODEL_LIST_AGENTS = frozenset({"opencode", "pi", "gemini", "copilot"})
@@ -250,8 +239,11 @@ def _enabled_agent_payload(tool: str, agent_config: dict) -> dict:
     if isinstance(model_config, dict):
         body = _model_config_payload(tool, model_config)
         if body:
-            variant = _AGENT_MODEL_CONFIG_VARIANT[tool]
-            config["model_config"] = {variant: body}
+            # The `AgentModelConfig` oneof field names are ucode's tool names verbatim (claude,
+            # codex, opencode, pi, gemini, copilot), so the tool doubles as the variant key. The
+            # server rejects a variant that doesn't match its agent (`validateAgentModelConfig`),
+            # and the round-trip through `normalize_managed_config` pins that alignment in tests.
+            config["model_config"] = {tool: body}
 
     entry: dict = {"agent": AGENT_TOOL_TO_ENUM[tool]}
     if config: