doctor: refactor OpenCode asset constants and problem discriminants

Consolidate OpenCode asset directory constants into opencode_asset module,
introduce typed ProblemKind discriminant to replace fragile summary-based
checks, cache repeated PathBuf computations, and extract shared label constants.

Co-authored-by: SCE <sce@crocoder.dev>

Author: davidabram

cli/src/services/default_paths.rs

@@ -185,6 +185,8 @@ pub(crate) mod opencode_asset {
     pub const RUNTIME_FILE: &str = "runtime.ts";
     pub const LIB_DIR: &str = "lib";
     pub const PRESET_CATALOG: &str = "bash-policy-presets.json";
+    pub const OPENCODE_AGENT_DIR: &str = "agent";
+    pub const OPENCODE_COMMAND_DIR: &str = "command";
     pub const SKILLS_DIR: &str = "skills";
     pub const AGENTS_DIR: &str = "agents";
 }