fix(learning): drop retrospective.system_prompt (no prompt file in this repo)

The retrospective block pointed system_prompt at prompts/retrospective_system.md,
which does not exist in this repo (only databricks-sql-kernel ships one; the
sql-python reference correctly omits the key). The engine treats a set-but-missing
system_prompt as a HARD ERROR, so the daily cron would fail every run.

Drop the key so the flow uses the engine's built-in base prompt
(engineer_prompts.RETRO_SYSTEM_PROMPT) — matching databricks-sql-python and the
odbc learning PR. Also corrects the config comment, which wrongly claimed the file
was "present in this repo."

Caught by peco-review-bot on the sibling PRs.

Co-authored-by: Isaac
Signed-off-by: eric-wang-1990 <e.wang@databricks.com>

Author: eric-wang-1990

.bot/config.yaml

@@ -116,11 +116,14 @@ flow: bug-fix
 # engine gathers ITSELF per PR) plus engineer-bot author runs, and if the model
 # finds a durable, reusable learning, opens/updates a single rolling PR appending a
 # dated section to log_path. Human-gated — never commits the canonical log directly.
-# Omitting this block makes the retrospective a no-op. The system prompt is the
-# engine's built-in base; prompts/retrospective_system.md (present in this repo) is
-# appended as additive guidance.
+# Omitting this block makes the retrospective a no-op.
+#
+# `system_prompt` is intentionally OMITTED: this repo ships no
+# prompts/retrospective_system.md, so the engine's built-in base prompt is used.
+# (A set-but-missing system_prompt file would be a hard error; an UNSET key uses
+# the base. Add the key later only if you write a repo-specific additive prompt.)
+# The learning log is created on first write — no seed file needed.
 retrospective:
-  system_prompt: prompts/retrospective_system.md
   log_path: .claude/knowledge/learning-log.md   # MUST match author.knowledge_log above
   branch_prefix: ai/learning-pr-
   pr_label: engineer-bot-learning