feat(agents): make OmO default agent via build name hack

- Set build agent's display name to 'OmO' (keeps builtIn: true priority)
- Add OmO as subagent (actual execution target when selected)
- Remove explicit tools list from OmO agent (inherit all)
- Rename omo_agent.disable_build to omo_agent.disabled

This hack works around OpenCode's agent selection by key name.
TODO: Use config.default_agent when PR #5313 is released.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/omo.ts

@@ -366,41 +366,6 @@ export const omoAgent: AgentConfig = {
     budgetTokens: 32000,
   },
   maxTokens: 128000,
-  tools: {
-    read: true,
-    write: true,
-    edit: true,
-    multiedit: true,
-    patch: true,
-    glob: true,
-    grep: true,
-    list: true,
-    bash: true,
-    batch: true,
-    webfetch: true,
-    websearch: true,
-    codesearch: true,
-    todowrite: true,
-    todoread: true,
-    task: true,
-    lsp_hover: true,
-    lsp_goto_definition: true,
-    lsp_find_references: true,
-    lsp_document_symbols: true,
-    lsp_workspace_symbols: true,
-    lsp_diagnostics: true,
-    lsp_rename: true,
-    lsp_prepare_rename: true,
-    lsp_code_actions: true,
-    lsp_code_action_resolve: true,
-    lsp_servers: true,
-    ast_grep_search: true,
-    ast_grep_replace: true,
-    skill: true,
-    call_omo_agent: true,
-    background_task: true,
-    background_output: true,
-  },
   prompt: OMO_SYSTEM_PROMPT,
   color: "#00CED1",
 }