Add hooks system with pi.send() for external message injection

- Hook discovery from ~/.pi/agent/hooks/, .pi/hooks/, --hook flag
- Events: session_start, session_switch, agent_start/end, turn_start/end, tool_call, tool_result, branch
- tool_call can block execution, tool_result can modify results
- pi.send(text, attachments?) to inject messages from external sources
- UI primitives: ctx.ui.select/confirm/input/notify
- Context: ctx.exec(), ctx.cwd, ctx.sessionFile, ctx.hasUI
- Docs shipped with npm package and binary builds
- System prompt references docs folder

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

@@ -888,6 +888,8 @@ export class AgentSession {
 	 * Listeners are preserved and will continue receiving events.
 	 */
 	async switchSession(sessionPath: string): Promise<void> {
+		const previousSessionFile = this.sessionFile;
+
 		this._disconnectFromAgent();
 		await this.abort();
 		this._queuedMessages = [];
@@ -895,6 +897,17 @@ export class AgentSession {
 		// Set new session
 		this.sessionManager.setSessionFile(sessionPath);
 
+		// Emit session_switch event
+		if (this._hookRunner) {
+			this._hookRunner.setSessionFile(sessionPath);
+			await this._hookRunner.emit({
+				type: "session_switch",
+				newSessionFile: sessionPath,
+				previousSessionFile,
+				reason: "switch",
+			});
+		}
+
 		// Reload messages
 		const loaded = loadSessionFromEntries(this.sessionManager.loadEntries());
 		this.agent.replaceMessages(loaded.messages);
@@ -928,6 +941,7 @@ export class AgentSession {
 	 *   - skipped: True if a hook requested to skip conversation restore
 	 */
 	async branch(entryIndex: number): Promise<{ selectedText: string; skipped: boolean }> {
+		const previousSessionFile = this.sessionFile;
 		const entries = this.sessionManager.loadEntries();
 		const selectedEntry = entries[entryIndex];
 
@@ -956,6 +970,17 @@ export class AgentSession {
 		const newSessionFile = this.sessionManager.createBranchedSessionFromEntries(entries, entryIndex);
 		this.sessionManager.setSessionFile(newSessionFile);
 
+		// Emit session_switch event
+		if (this._hookRunner) {
+			this._hookRunner.setSessionFile(newSessionFile);
+			await this._hookRunner.emit({
+				type: "session_switch",
+				newSessionFile,
+				previousSessionFile,
+				reason: "branch",
+			});
+		}
+
 		// Reload
 		const loaded = loadSessionFromEntries(this.sessionManager.loadEntries());
 		this.agent.replaceMessages(loaded.messages);