Implement the /publish slash command to push local sites to WP.com (#4043)

## Related issues

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the issue,
for example, it requires a separate deployment to production, avoid
using the "Fixes" keyword and use "Related to" instead.
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Fixes STU-1552

## How AI was used in this PR

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reviewed the code yourself.
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## Proposed Changes

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- How does it affect the user (new capability, fix, performance,
accessibility, etc.)?
- Note any user-visible behavior changes or trade-offs.

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This PR adds a `/publish` command for publishing local sites to WP.com.

## Testing Instructions

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Add as many details as possible to help others reproduce the issue and
test the fix.
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* Pull the changes from this branch
* Build CLI with `npm run cli:build`
* Launch Studio code with `node apps/cli/dist/cli/main.mjs code`
* Ensure that you have no sites selected first
* Type `publish`
* Confirm that you are encouraged to select a site

<img width="738" height="204" alt="Screenshot 2026-07-02 at 10 46 23 AM"
src="https://github.com/user-attachments/assets/70596d4a-6a28-40ca-bdc6-d1f7afd37482"
/>

* Once the site is selected, type `/publish` again
* Select a remote site to publish to
* Start push process 
* Confirm that it goes through
* Then, test with `/publish` command for an account that does not have
any remote sites (you can use this diff and will need to rebuild CLI):

```diff --git a/apps/cli/ai/slash-commands.ts b/apps/cli/ai/slash-commands.ts
index e56a72b..ebf34e5de 100644
--- a/apps/cli/ai/slash-commands.ts
+++ b/apps/cli/ai/slash-commands.ts
@@ -515,7 +515,7 @@ export const AI_CHAT_SLASH_COMMANDS: SlashCommandDef[] = [
                                ctx.ui.showProgress( __( 'Fetching your WordPress.com sites…' ) );
                                ctx.ui.setBusy( true );
 
-                               const remoteSites = await fetchSyncableSites( token.accessToken );
+                               const remoteSites: Awaited< ReturnType< typeof fetchSyncableSites > > = []; // TODO: remove mock
                                const syncable = remoteSites.filter( ( s ) => s.syncSupport === 'syncable' );
 
                                if ( syncable.length === 0 ) {
```
* Confirm it opens a link in the browser if you have no WP.com sites:

<img width="751" height="243" alt="Screenshot 2026-07-02 at 11 23 36 AM"
src="https://github.com/user-attachments/assets/bdc9f6cb-371e-491b-bc99-f3a055197f79"
/>
* Try using the `/publish` command when you are logged out and confirm
that you are asked to login first


## Pre-merge Checklist

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- [ ] Have you checked for TypeScript, React or other console errors?

---------

Co-authored-by: Kateryna Kodonenko <kateryna@automattic.com>

Author: katinthehatsite

apps/cli/ai/slash-commands.ts

@@ -10,8 +10,10 @@ import { runCommand as runLoginCommand } from 'cli/commands/auth/login';
 import { runCommand as runLogoutCommand } from 'cli/commands/auth/logout';
 import { runCommand as runCreatePreviewCommand } from 'cli/commands/preview/create';
 import { runCommand as runUpdatePreviewCommand } from 'cli/commands/preview/update';
+import { runCommand as runPushCommand } from 'cli/commands/push';
 import { openBrowser } from 'cli/lib/browser';
 import { getSnapshotsFromConfig, isSnapshotExpired } from 'cli/lib/snapshots';
+import { fetchSyncableSites } from 'cli/lib/sync-api';
 import { LoggerError } from 'cli/logger';
 import { loadRemoteSessionConfig } from 'cli/remote-session/config';
 import { DaemonAlreadyRunningError, startDaemon, stopDaemon } from 'cli/remote-session/daemon';
@@ -493,6 +495,98 @@ export const AI_CHAT_SLASH_COMMANDS: SlashCommandDef[] = [
 			return 'continue';
 		},
 	},
+	{
+		name: 'publish',
+		description: __( 'Publish the active site to WordPress.com' ),
+		handler: async ( _prompt, ctx ) => {
+			const site = ctx.ui.activeSite;
+			if ( ! site ) {
+				ctx.ui.showInfo( __( 'No site selected. Use ↓ to select a site first.' ) );
+				return 'continue';
+			}
+
+			const token = await readAuthToken();
+			if ( ! token ) {
+				ctx.ui.showInfo( __( 'WordPress.com login required. Use /login to authenticate.' ) );
+				return 'continue';
+			}
+
+			try {
+				ctx.ui.showProgress( __( 'Fetching your WordPress.com sites…' ) );
+				ctx.ui.setBusy( true );
+
+				const remoteSites = await fetchSyncableSites( token.accessToken );
+				const syncable = remoteSites.filter( ( s ) => s.syncSupport === 'syncable' );
+
+				if ( syncable.length === 0 ) {
+					ctx.ui.setBusy( false );
+					const checkoutUrl = new URL( 'https://wordpress.com/setup/new-hosted-site' );
+					checkoutUrl.searchParams.set( 'ref', 'studio' );
+					checkoutUrl.searchParams.set( 'section', 'studio-sync' );
+					checkoutUrl.searchParams.set( 'showDomainStep', 'true' );
+					checkoutUrl.searchParams.set( 'new', site.name );
+
+					await openBrowser( checkoutUrl.toString() );
+					ctx.ui.showInfo(
+						__(
+							'No WordPress.com sites found. Opening WordPress.com to create one — once your site is ready, run /publish again.'
+						)
+					);
+					return 'continue';
+				}
+
+				// Let the user pick which site to publish to.
+				const siteOptions = syncable.map( ( s ) => ( {
+					label: s.name,
+					description: s.url,
+				} ) );
+				ctx.ui.setBusy( false );
+				const answer = await ctx.ui.askUser( [
+					{
+						question: __( 'Select a WordPress.com site to publish to' ),
+						options: siteOptions,
+					},
+				] );
+				const selectedName = Object.values( answer )[ 0 ] as string;
+				const remoteSite = syncable.find( ( s ) => s.name === selectedName );
+				if ( ! remoteSite ) {
+					ctx.ui.showInfo( __( 'No site selected.' ) );
+					return 'continue';
+				}
+
+				ctx.ui.showProgress(
+					sprintf( __( 'Publishing to %s… this may take several minutes.' ), remoteSite.name )
+				);
+				ctx.ui.setBusy( true );
+
+				const result = await captureCommandOutput( () =>
+					runPushCommand( site.path, [ 'all' ], String( remoteSite.id ) )
+				);
+
+				ctx.ui.setBusy( false );
+
+				if ( result.exitCode ) {
+					ctx.ui.showError( result.consoleOutput || __( 'Failed to publish site.' ) );
+				} else {
+					ctx.ui.showSuccess(
+						sprintf( __( 'Published to %s' ), remoteSite.url ) + '\n\n   ' + remoteSite.url
+					);
+				}
+			} catch ( error ) {
+				ctx.ui.setBusy( false );
+				if ( isPromptAbortError( error ) ) {
+					ctx.ui.showInfo( __( 'Publish canceled.' ) );
+					return 'continue';
+				}
+				if ( error instanceof LoggerError ) {
+					ctx.ui.showError( error.message );
+				} else {
+					ctx.ui.showError( __( 'Failed to publish site.' ) );
+				}
+			}
+			return 'continue';
+		},
+	},
 	{
 		name: 'remote-session',
 		description: __( 'Manage the Telegram remote-session daemon (start, stop)' ),