story-kit: start 176_bug_stories_moved_to_current_get_supervisor_instead_of_coder

.story_kit/work/2_current/176_bug_stories_moved_to_current_get_supervisor_instead_of_coder.md

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+---
+name: "Stories moved to Current get supervisor instead of coder"
+---
+
+# Bug 176: Stories moved to Current get supervisor instead of coder
+
+## Description
+
+When start_agent is called without an explicit agent_name, it defaults to the first agent in the project.toml roster — which is supervisor. This means stories entering the Current stage get a supervisor assigned instead of a coder, requiring manual intervention to stop the supervisor, remove the worktree, and restart with a coder. This thrashing has also caused worktree corruption (broken git metadata) leading to agents escaping their worktree and editing master directly.
+
+## How to Reproduce
+
+1. Call start_agent with only a story_id, no agent_name
+2. Observe that supervisor is assigned instead of a coder
+
+## Actual Result
+
+Supervisor agent is spawned because it's the first entry in the agent roster.
+
+## Expected Result
+
+A coder agent should be assigned to stories in the Current stage. Supervisor should only be invoked explicitly.
+
+## Acceptance Criteria
+
+- [ ] start_agent without an agent_name assigns a coder (not supervisor) for stories in Current
+- [ ] Supervisor is only started when explicitly requested by name
+- [ ] If no coder is available, the error message says so rather than silently falling back to supervisor