feat: add unblock command and MCP tool to reset blocked stories

- Add `unblock` bot command (chat + web UI slash command) that clears the
  `blocked` flag and resets `retry_count` to 0 in story front matter
- Works across all pipeline stages (1_backlog through 6_archived)
- Returns confirmation with story name and ID, or clear error if story
  is not found or not blocked
- Expose `unblock_story` MCP tool for programmatic use by agents
- Make `chat::commands::unblock` module pub(crate) so story_tools can
  call `unblock_by_number`
- Add 8 unit tests covering registration, validation, core logic, and
  edge cases (not-found, not-blocked, any stage, story ID in response)
- Update MCP tools list test: 49 → 50 tools

server/src/http/mcp/mod.rs

@@ -1006,6 +1006,20 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                         "required": ["story_id", "target_stage"]
                     }
                 },
+                {
+                    "name": "unblock_story",
+                    "description": "Clear the blocked flag and reset retry_count to 0 on a work item. Use this when an agent is stuck and needs to be restarted from a clean state.",
+                    "inputSchema": {
+                        "type": "object",
+                        "properties": {
+                            "story_id": {
+                                "type": "string",
+                                "description": "Work item identifier (filename stem, e.g. '42_story_my_feature')"
+                            }
+                        },
+                        "required": ["story_id"]
+                    }
+                },
                 {
                     "name": "run_command",
                     "description": "Execute a shell command in an agent's worktree directory. The working_dir must be inside .storkit/worktrees/. Returns stdout, stderr, exit_code, and timed_out. Supports SSE streaming (send Accept: text/event-stream) for long-running commands. Dangerous commands (rm -rf /, sudo, etc.) are blocked.",
@@ -1230,6 +1244,8 @@ async fn handle_tools_call(
         "delete_story" => story_tools::tool_delete_story(&args, ctx).await,
         // Arbitrary pipeline movement
         "move_story" => diagnostics::tool_move_story(&args, ctx),
+        // Unblock story
+        "unblock_story" => story_tools::tool_unblock_story(&args, ctx),
         // Shell command execution
         "run_command" => shell_tools::tool_run_command(&args, ctx).await,
         // Git operations
@@ -1350,6 +1366,7 @@ mod tests {
         assert!(names.contains(&"rebuild_and_restart"));
         assert!(names.contains(&"get_token_usage"));
         assert!(names.contains(&"move_story"));
+        assert!(names.contains(&"unblock_story"));
         assert!(names.contains(&"delete_story"));
         assert!(names.contains(&"run_command"));
         assert!(names.contains(&"git_status"));
@@ -1359,7 +1376,7 @@ mod tests {
         assert!(names.contains(&"git_log"));
         assert!(names.contains(&"status"));
         assert!(names.contains(&"loc_file"));
-        assert_eq!(tools.len(), 50);
+        assert_eq!(tools.len(), 51);
     }
 
     #[test]