execute: parallel steps, pipeline stages, language selection docs

- prompts/02_tools.md: parallel execution and language selection sections
- skills/execute-pipe/SKILL.md: new skill — pipeline patterns, parallel
  fan-out, ETL pattern, language selection table, when to load guidance
- core submodule updated to parallel execute_code/execute_pipe

core

@@ -1 +1 @@
-Subproject commit 54f20bce023ca238ffd7c56f461d60cdfb3d8771
+Subproject commit cf25268d80ef23925e43b18248a63d52668076e4

prompts/02_tools.md

@@ -1,6 +1,6 @@
 # Tools
 
-Tool scripts live at `{{.OllieMountPath}}/t`. Run them with `execute_tool` or as steps in an `execute_pipe` pipeline.
+Tool scripts live at `{{.OllieMountPath}}/t`. Run them with `execute_tool` or as stages in an `execute_pipe` pipeline.
 
 Supported languages are detected from the script's shebang line:
 - `#!/usr/bin/env bash` — runs with `bash -c`
@@ -15,3 +15,15 @@ args: <required> [optional]
 ```
 
 Search before acting: `grep -A2 "keyword" {{.OllieMountPath}}/t/idx`
+
+## Parallel execution
+
+`execute_code` accepts a `steps` array. Multiple steps run concurrently; results are returned in submission order. Single-step calls return raw output.
+
+`execute_pipe` stages may be `{parallel: [...]}` to fan out concurrently within a pipeline. All parallel steps should produce the same output schema so their concatenated output is coherent for the next stage. Load the `execute-pipe` skill for full pipeline patterns and the ETL pattern for disparate schemas.
+
+## Language selection
+
+For inline code steps (`{code}`), set `language` to choose the interpreter (default: `bash`). Supported: `bash`, `python3`, `perl`, `lua`, `awk`, `sed`, `jq`, `ed`, `expect`, `bc`.
+
+For tool steps (`{tool}`), the interpreter is determined by the script's shebang line — `language` is ignored.