fix(coding-agent): make session path traversal linear

Avoid quadratic path construction when walking deep session branches. On a 600k-entry pathological session, path construction improved from about 20.3s with Array.unshift() to about 35ms with push() plus reverse().

Refs #5804.

packages/coding-agent/CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ### Fixed
 
+- Fixed deep session branches taking quadratic time to build context or branch paths ([#5909](https://github.com/earendil-works/pi/issues/5909)).
 - Fixed fuzzy `edit` matches to preserve untouched line blocks instead of rewriting the whole file through normalized content ([#5899](https://github.com/earendil-works/pi/issues/5899)).
 - Fixed bash commands through legacy WSL `bash.exe` to pass scripts over stdin so shell variables expand in the target bash ([#5893](https://github.com/earendil-works/pi/issues/5893)).
 - Fixed `/model` to hide GitHub Copilot models that are unavailable to the authenticated account ([#5897](https://github.com/earendil-works/pi/issues/5897)).

packages/coding-agent/src/core/session-manager.ts

@@ -357,9 +357,10 @@ export function buildSessionContext(
 	const path: SessionEntry[] = [];
 	let current: SessionEntry | undefined = leaf;
 	while (current) {
-		path.unshift(current);
+		path.push(current);
 		current = current.parentId ? byId.get(current.parentId) : undefined;
 	}
+	path.reverse();
 
 	// Extract settings and find compaction
 	let thinkingLevel = "off";
@@ -1152,9 +1153,10 @@ export class SessionManager {
 		const startId = fromId ?? this.leafId;
 		let current = startId ? this.byId.get(startId) : undefined;
 		while (current) {
-			path.unshift(current);
+			path.push(current);
 			current = current.parentId ? this.byId.get(current.parentId) : undefined;
 		}
+		path.reverse();
 		return path;
 	}