chore(examples): add qa_ping smoke workflow; ignore tui-go binary and qa/ scratch

qa_ping is a minimal single-stage file_write workflow used for live QA smoke
runs (writes 'pong' into the qa/ scratch repo). The qa/ workspace itself is a
nested git repo and stays untracked; the compiled tui-go binary is ignored.

examples/workflows/prompts/qa_ping.md

@@ -0,0 +1 @@
+Write the single word 'pong' to the file qa/sample-repo/ping.txt using file_write, then call stage_complete. Do not explain.

examples/workflows/qa_ping.toml

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+id = "qa_ping"
+start = "ping"
+
+[[stages]]
+id = "ping"
+prompt = "prompts/qa_ping.md"
+produces = [{ name = "ping_result", kind = "file_written" }]
+allowed_tools = ["file_write"]
+token_budget = 1024
+max_retries = 1
+
+[[transitions]]
+id = "ping-to-done"
+from = "ping"
+to = "done"
+condition_type = "always_true"