Story 44: Agent Completion Report via MCP

- report_completion MCP tool for agents to signal done
- Rejects if worktree has uncommitted changes
- Runs acceptance gates (clippy, tests) automatically
- Stores completion status on agent record
- 10 new tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/config.rs

@@ -59,7 +59,11 @@ fn default_agent_command() -> String {
 }
 
 fn default_agent_prompt() -> String {
-    "Read .story_kit/README.md, then pick up story {{story_id}}".to_string()
+    "You are working in a git worktree on story {{story_id}}. \
+     Read .story_kit/README.md to understand the dev process, then pick up the story. \
+     When all work is committed, call report_completion with story_id='{{story_id}}', \
+     agent_name='{{agent_name}}', and a brief summary as your final action."
+        .to_string()
 }
 
 /// Legacy config format with `agent` as an optional single table (`[agent]`).
@@ -187,10 +191,12 @@ impl ProjectConfig {
         };
 
         let bb = base_branch.unwrap_or("master");
+        let aname = agent.name.as_str();
         let render = |s: &str| {
             s.replace("{{worktree_path}}", worktree_path)
                 .replace("{{story_id}}", story_id)
                 .replace("{{base_branch}}", bb)
+                .replace("{{agent_name}}", aname)
         };
 
         let command = render(&agent.command);