feat(coding-agent): add session_directory extension event

Adds a session_directory extension event that fires before session manager
creation, allowing extensions to customize the session directory path based
on cwd, git branch, or other context.

- Extensions can return a custom sessionDir in the event handler
- CLI --session-dir flag takes precedence over extension-provided paths
- If multiple extensions return a sessionDir, the last one wins
- Enables implementing branch-based sessions as an extension instead of core feature

This provides the extension point needed to implement git branch-based
session directories without adding the complexity to core.

Closes #1729

packages/coding-agent/src/core/extensions/index.ts

@@ -114,6 +114,8 @@ export type {
 	SessionBeforeTreeEvent,
 	SessionBeforeTreeResult,
 	SessionCompactEvent,
+	SessionDirectoryEvent,
+	SessionDirectoryResult,
 	SessionEvent,
 	SessionForkEvent,
 	SessionShutdownEvent,

packages/coding-agent/src/core/extensions/runner.ts

@@ -851,6 +851,40 @@ export class ExtensionRunner {
 		return { skillPaths, promptPaths, themePaths };
 	}
 
+	/** Emit session_directory event. Returns custom session directory from extensions (last one wins). */
+	async emitSessionDirectory(cwd: string, cliSessionDir: string | undefined): Promise<string | undefined> {
+		const ctx = this.createContext();
+		let customSessionDir: string | undefined;
+
+		for (const ext of this.extensions) {
+			const handlers = ext.handlers.get("session_directory");
+			if (!handlers || handlers.length === 0) continue;
+
+			for (const handler of handlers) {
+				try {
+					const event = { type: "session_directory" as const, cwd, cliSessionDir };
+					const handlerResult = await handler(event, ctx);
+					const result = handlerResult as { sessionDir?: string } | undefined;
+
+					if (result?.sessionDir) {
+						customSessionDir = result.sessionDir;
+					}
+				} catch (err) {
+					const message = err instanceof Error ? err.message : String(err);
+					const stack = err instanceof Error ? err.stack : undefined;
+					this.emitError({
+						extensionPath: ext.path,
+						event: "session_directory",
+						error: message,
+						stack,
+					});
+				}
+			}
+		}
+
+		return customSessionDir;
+	}
+
 	/** Emit input event. Transforms chain, "handled" short-circuits. */
 	async emitInput(text: string, images: ImageContent[] | undefined, source: InputSource): Promise<InputEventResult> {
 		const ctx = this.createContext();

packages/coding-agent/src/core/extensions/types.ts

@@ -388,6 +388,14 @@ export interface ResourcesDiscoverResult {
 // Session Events
 // ============================================================================
 
+/** Fired before session manager creation to allow custom session directory resolution */
+export interface SessionDirectoryEvent {
+	type: "session_directory";
+	cwd: string;
+	/** CLI-provided session directory (if any) */
+	cliSessionDir: string | undefined;
+}
+
 /** Fired on initial session load */
 export interface SessionStartEvent {
 	type: "session_start";
@@ -472,6 +480,7 @@ export interface SessionTreeEvent {
 }
 
 export type SessionEvent =
+	| SessionDirectoryEvent
 	| SessionStartEvent
 	| SessionBeforeSwitchEvent
 	| SessionSwitchEvent
@@ -866,6 +875,11 @@ export interface BeforeAgentStartEventResult {
 	systemPrompt?: string;
 }
 
+export interface SessionDirectoryResult {
+	/** Custom session directory path. If multiple extensions return this, the last one wins. */
+	sessionDir?: string;
+}
+
 export interface SessionBeforeSwitchResult {
 	cancel?: boolean;
 }
@@ -936,6 +950,7 @@ export interface ExtensionAPI {
 	// =========================================================================
 
 	on(event: "resources_discover", handler: ExtensionHandler<ResourcesDiscoverEvent, ResourcesDiscoverResult>): void;
+	on(event: "session_directory", handler: ExtensionHandler<SessionDirectoryEvent, SessionDirectoryResult>): void;
 	on(event: "session_start", handler: ExtensionHandler<SessionStartEvent>): void;
 	on(
 		event: "session_before_switch",