core/agent+config: configurable compact prompts

Add agent config compact_system_prompt, compact_user_prefix, and
compact_seed_text; wire through summarize and seed messages. Fix default
user prefix to include {transcript}; use safe seed substitution so braces
in summaries do not break formatting.

src/plyngent/agent/compact.py

@@ -33,6 +33,13 @@ _SUMMARY_SYSTEM = (
 _SUMMARY_USER_PREFIX = (
     "Summarize the following conversation for continued agent work. "
     "Output only the summary (no preamble).\n\n--- transcript ---\n"
+    "{transcript}"
+)
+
+_SEED_MESSAGE_TEMPLATE = (
+    "Conversation summary (compacted from {src}):\n\n"
+    "{summary}\n\n"
+    "Continue from this summary. Prefer not to re-ask for information already covered."
 )
 
 
@@ -88,8 +95,21 @@ async def summarize_messages(
     temperature: float | None = 0.2,
     prompt_tokens_hint: int | None = None,
     sent_estimate_tokens: int | None = None,
+    system_prompt: str | None = None,
+    user_prefix: str | None = None,
 ) -> str:
-    """Soft-compact history and ask the model for a dense summary (no tools)."""
+    """Soft-compact history and ask the model for a dense summary (no tools).
+
+    Parameters
+    ----------
+    system_prompt:
+        Override for the summarizer system prompt.
+        ``None`` or empty string uses the built-in default.
+    user_prefix:
+        Override for the user-message prefix (appended before the transcript).
+        ``None`` or empty string uses the built-in default.
+        The placeholder ``{transcript}`` is substituted with the rendered transcript.
+    """
     if not messages:
         msg = "nothing to compact"
         raise ValueError(msg)
@@ -102,10 +122,19 @@ async def summarize_messages(
     if not transcript.strip():
         msg = "nothing to compact"
         raise ValueError(msg)
+
+    sys_content = system_prompt.strip() if system_prompt else _SUMMARY_SYSTEM
+    user_prefix_resolved = user_prefix.strip() if user_prefix else _SUMMARY_USER_PREFIX
+    if "{transcript}" in user_prefix_resolved:
+        user_content = user_prefix_resolved.replace("{transcript}", transcript)
+    else:
+        # Custom prefix without placeholder: append transcript (legacy/default shape).
+        user_content = user_prefix_resolved + transcript
+
     param = ChatCompletionsParam(
         messages=[
-            SystemChatMessage(content=_SUMMARY_SYSTEM),
-            UserChatMessage(content=_SUMMARY_USER_PREFIX + transcript),
+            SystemChatMessage(content=sys_content),
+            UserChatMessage(content=user_content),
         ],
         model=model,
         temperature=temperature if temperature is not None else UNSET,
@@ -126,20 +155,26 @@ def build_compacted_seed_messages(
     *,
     system_prompt: str | None = None,
     source_session_id: int | None = None,
+    seed_text: str | None = None,
 ) -> list[AnyChatMessage]:
     """Messages to seed a new session after compact.
 
     Summary is an assistant message so history does not end with a user turn
     (which would look like an incomplete /retry-able request).
+
+    Parameters
+    ----------
+    seed_text:
+        Override template for the seed assistant message body.
+        ``None`` or empty string uses the built-in default.
+        Placeholders ``{src}`` and ``{summary}`` are substituted.
     """
     out: list[AnyChatMessage] = []
     if system_prompt:
         out.append(SystemChatMessage(content=system_prompt))
     src = f"session {source_session_id}" if source_session_id is not None else "prior session"
-    body = (
-        f"Conversation summary (compacted from {src}):\n\n"
-        f"{summary}\n\n"
-        "Continue from this summary. Prefer not to re-ask for information already covered."
-    )
+    template = seed_text.strip() if seed_text else _SEED_MESSAGE_TEMPLATE
+    # Use replace (not str.format) so braces inside the model summary are safe.
+    body = template.replace("{src}", src).replace("{summary}", summary)
     out.append(AssistantChatMessage(content=body))
     return out

src/plyngent/cli/editor.py

@@ -30,6 +30,11 @@ _MINIMAL_CONFIG = """\
 # confirm_destructive = true
 # path_denylist = ["/secrets/", ".ssh/"]
 # max_context_tokens = 200000
+#
+# # Optional compact prompts (empty = use built-in defaults).
+# # compact_system_prompt = ""
+# # compact_user_prefix = "Summarize:\n\n{transcript}"
+# # compact_seed_text = "Compacted from {src}:\n\n{summary}"
 
 # [providers.example]
 # preset = "openai-compatible"

src/plyngent/cli/state.py

@@ -303,6 +303,8 @@ class ReplState:
             max_context_tokens=self.agent.max_context_tokens,
             prompt_tokens_hint=hint,
             sent_estimate_tokens=sent_est,
+            system_prompt=self.config.agent_config.compact_system_prompt or None,
+            user_prefix=self.config.agent_config.compact_user_prefix or None,
         )
         session_name = name or f"compact-from-{old_id}"
         await self.new_session(name=session_name)
@@ -315,6 +317,7 @@ class ReplState:
             summary,
             system_prompt=self.agent.system_prompt,
             source_session_id=old_id,
+            seed_text=self.config.agent_config.compact_seed_text or None,
         )
         self.agent.messages = list(seed)
         for message in seed:

src/plyngent/config/models.py

@@ -20,6 +20,11 @@ class AgentConfig(Struct, omit_defaults=True):
     path_denylist: list[str] = field(default_factory=list)
     max_context_tokens: int = 200_000
 
+    # Compact / summarisation prompts (empty = use built-in defaults).
+    compact_system_prompt: str = ""
+    compact_user_prefix: str = ""
+    compact_seed_text: str = ""
+
 
 class ModelConfig(Struct, omit_defaults=True):
     """Capability flags for a model within a provider."""