fix: revert opencode agent and fix lefthook config (#22)

## Summary
- Reverts the opencode agent support added in #18 (duplicate entry)
- Fixes lefthook pre-commit config: removes `parallel: true` and adds
`stage_fixed: true` to fmt job

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Code <noreply@anthropic.com>

Author: alecthomas

lefthook.yml

@@ -4,9 +4,9 @@ output:
   - failure
 
 pre-commit:
-  parallel: true
   jobs:
     - name: fmt
+      stage_fixed: true
       run: just fmt
 pre-push:
   parallel: true

src/agent.rs

@@ -97,13 +97,6 @@ pub const KNOWN_AGENTS: &[Agent] = &[
         email: "opencode <noreply@opencode.ai>",
         ..Agent::default()
     },
-    Agent {
-        process_names: &["opencode"],
-        env_vars: &[],
-        email: "OpenCode <noreply@opencode.ai>",
-        breadcrumb_dir: None,
-        breadcrumb_ext: None,
-    },
 ];
 
 impl Agent {

src/main.rs

@@ -316,7 +316,6 @@ mod tests {
         assert!(Agent::find_by_name("/opt/homebrew/bin/amp").is_some());
         assert!(Agent::find_by_name("gemini").is_some());
         assert!(Agent::find_by_name("goose").is_some());
-        assert!(Agent::find_by_name("opencode").is_some());
         assert!(Agent::find_by_name("unknown").is_none());
     }