feat: replace Git-DAG timeline with flat snapshot browser

Replace HistoryTree (DAG with branches, Graphviz rendering) with a flat
chronological SnapshotTimeline. New UI features: split-view layout,
snapshot compare/diff, cherry-pick restore of individual sequences or
fields, auto-snapshots with debounce, and pin/filter support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

snapshot_timeline.py

@@ -0,0 +1,184 @@
+import time
+import uuid
+from typing import Any
+
+KEY_PROMPT_HISTORY = "prompt_history"
+
+
+class SnapshotTimeline:
+    """Flat chronological snapshot list — replaces the old HistoryTree DAG."""
+
+    def __init__(self, raw_data: dict[str, Any]) -> None:
+        # Detect and migrate old HistoryTree format
+        if "nodes" in raw_data and "branches" in raw_data:
+            self._migrate_from_tree(raw_data)
+        elif KEY_PROMPT_HISTORY in raw_data and isinstance(raw_data[KEY_PROMPT_HISTORY], list):
+            self._migrate_legacy(raw_data[KEY_PROMPT_HISTORY])
+        else:
+            self.snapshots: dict[str, dict[str, Any]] = raw_data.get("snapshots", {})
+            self.current_id: str | None = raw_data.get("current_id", None)
+
+    # ------------------------------------------------------------------
+    # Migration
+    # ------------------------------------------------------------------
+
+    def _migrate_from_tree(self, raw_data: dict[str, Any]) -> None:
+        """Flatten old HistoryTree nodes into snapshot list, discarding DAG info."""
+        self.snapshots = {}
+        nodes = raw_data.get("nodes", {})
+        for nid, node in nodes.items():
+            self.snapshots[nid] = {
+                "id": nid,
+                "timestamp": node.get("timestamp", time.time()),
+                "note": node.get("note", "Migrated"),
+                "pinned": False,
+                "auto": False,
+                "seq_count": self._count_seqs(node.get("data")),
+            }
+            # Preserve snapshot data if present
+            if "data" in node and node["data"]:
+                self.snapshots[nid]["data"] = node["data"]
+        self.current_id = raw_data.get("head_id")
+
+    def _migrate_legacy(self, old_list: list[dict[str, Any]]) -> None:
+        """Convert ancient prompt_history list into snapshots."""
+        self.snapshots = {}
+        self.current_id = None
+        for item in reversed(old_list):
+            sid = self._make_id()
+            self.snapshots[sid] = {
+                "id": sid,
+                "timestamp": time.time(),
+                "note": item.get("note", "Legacy Import"),
+                "pinned": False,
+                "auto": False,
+                "seq_count": self._count_seqs(item),
+                "data": item,
+            }
+            self.current_id = sid
+
+    # ------------------------------------------------------------------
+    # Core operations
+    # ------------------------------------------------------------------
+
+    def record(self, data: dict[str, Any], note: str = "Snapshot",
+               auto: bool = False) -> str:
+        """Create a new snapshot and return its ID."""
+        sid = self._make_id()
+        self.snapshots[sid] = {
+            "id": sid,
+            "timestamp": time.time(),
+            "note": note,
+            "pinned": False,
+            "auto": auto,
+            "seq_count": self._count_seqs(data),
+            "data": data,
+        }
+        self.current_id = sid
+        return sid
+
+    def get_snapshot_data(self, snapshot_id: str) -> dict[str, Any] | None:
+        """Return the inline snapshot data if present."""
+        snap = self.snapshots.get(snapshot_id)
+        if snap:
+            return snap.get("data")
+        return None
+
+    def toggle_pin(self, snapshot_id: str) -> bool:
+        """Toggle pinned state, return new value."""
+        snap = self.snapshots.get(snapshot_id)
+        if snap:
+            snap["pinned"] = not snap.get("pinned", False)
+            return snap["pinned"]
+        return False
+
+    def delete(self, snapshot_id: str) -> None:
+        """Remove a snapshot."""
+        self.snapshots.pop(snapshot_id, None)
+        if self.current_id == snapshot_id:
+            # Fall back to most recent remaining
+            if self.snapshots:
+                self.current_id = max(
+                    self.snapshots.values(), key=lambda s: s["timestamp"]
+                )["id"]
+            else:
+                self.current_id = None
+
+    def strip_snapshots(self) -> None:
+        """Remove inline data from all snapshots (for slim JSON storage)."""
+        for snap in self.snapshots.values():
+            snap.pop("data", None)
+
+    # ------------------------------------------------------------------
+    # Serialization
+    # ------------------------------------------------------------------
+
+    def to_dict(self) -> dict[str, Any]:
+        return {
+            "snapshots": self.snapshots,
+            "current_id": self.current_id,
+        }
+
+    # ------------------------------------------------------------------
+    # Helpers
+    # ------------------------------------------------------------------
+
+    def _make_id(self) -> str:
+        for _ in range(10):
+            sid = str(uuid.uuid4())[:8]
+            if sid not in self.snapshots:
+                return sid
+        raise ValueError("Failed to generate unique snapshot ID after 10 attempts")
+
+    @staticmethod
+    def _count_seqs(data: dict | None) -> int:
+        if not data:
+            return 0
+        from utils import KEY_BATCH_DATA
+        batch = data.get(KEY_BATCH_DATA, [])
+        return len(batch) if isinstance(batch, list) else 0
+
+
+# ------------------------------------------------------------------
+# Diff function
+# ------------------------------------------------------------------
+
+def diff_snapshots(old_batch: list[dict], new_batch: list[dict]) -> list[dict]:
+    """Compare two batch lists by sequence_number, return per-sequence diffs.
+
+    Returns a list of dicts:
+        {
+            "seq_num": int,
+            "status": "unchanged" | "changed" | "added" | "removed",
+            "changes": [{"field": str, "old": Any, "new": Any}],
+        }
+    """
+    from utils import KEY_SEQUENCE_NUMBER
+
+    old_by_seq = {int(s.get(KEY_SEQUENCE_NUMBER, 0)): s for s in old_batch}
+    new_by_seq = {int(s.get(KEY_SEQUENCE_NUMBER, 0)): s for s in new_batch}
+
+    all_seqs = sorted(set(old_by_seq) | set(new_by_seq))
+    result = []
+
+    for seq_num in all_seqs:
+        old_item = old_by_seq.get(seq_num)
+        new_item = new_by_seq.get(seq_num)
+
+        if old_item and not new_item:
+            result.append({"seq_num": seq_num, "status": "removed", "changes": []})
+        elif new_item and not old_item:
+            result.append({"seq_num": seq_num, "status": "added", "changes": []})
+        else:
+            # Both exist — field-by-field comparison
+            all_keys = sorted(set(old_item) | set(new_item))
+            changes = []
+            for k in all_keys:
+                old_val = old_item.get(k)
+                new_val = new_item.get(k)
+                if old_val != new_val:
+                    changes.append({"field": k, "old": old_val, "new": new_val})
+            status = "changed" if changes else "unchanged"
+            result.append({"seq_num": seq_num, "status": status, "changes": changes})
+
+    return result