feat(coding-agent): add PI_SKIP_VERSION_CHECK env variable

Skip new version check at startup when set. Useful for Nix or other
package manager installs where npm install is not used.

Also documents PI_CODING_AGENT_DIR and other environment variables
in the README.

closes #549

Co-authored-by: Aos Dabbagh <25783780+aos@users.noreply.github.com>

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -489,6 +489,8 @@ export class InteractiveMode {
 	 * Check npm registry for a newer version.
 	 */
 	private async checkForNewVersion(): Promise<string | undefined> {
+		if (process.env.PI_SKIP_VERSION_CHECK) return undefined;
+
 		try {
 			const response = await fetch("https://registry.npmjs.org/@mariozechner/pi-coding-agent/latest");
 			if (!response.ok) return undefined;