feat: add mergify stack sync CLI command

mergify_cli/stack/cli.py

@@ -22,6 +22,7 @@
 from mergify_cli.stack import session as stack_session_mod
 from mergify_cli.stack import setup as stack_setup_mod
 from mergify_cli.stack import skill as stack_skill_mod
+from mergify_cli.stack import sync as stack_sync_mod
 
 
 def trunk_type(
@@ -504,6 +505,38 @@ def skill() -> None:
     click.echo(stack_skill_mod.get_skill_content(), nl=False)
 
 
+@stack.command(help="Sync the stack: fetch trunk, remove merged commits, rebase")
+@click.pass_context
+@click.option(
+    "--dry-run",
+    "-n",
+    is_flag=True,
+    default=False,
+    help="Show what would happen without making changes",
+)
+@click.option(
+    "--trunk",
+    "-t",
+    type=click.UNPROCESSED,
+    default=lambda: asyncio.run(utils.get_trunk()),
+    callback=trunk_type,
+    help="Change the target branch of the stack.",
+)
+@utils.run_with_asyncio
+async def sync(
+    ctx: click.Context,
+    *,
+    dry_run: bool,
+    trunk: tuple[str, str],
+) -> None:
+    await stack_sync_mod.stack_sync(
+        github_server=ctx.obj["github_server"],
+        token=ctx.obj["token"],
+        trunk=trunk,
+        dry_run=dry_run,
+    )
+
+
 @stack.command(name="list", help="List the stack's commits and their associated PRs")
 @click.pass_context
 @click.option(

mergify_cli/stack/skill.md

@@ -33,10 +33,13 @@ A branch is a stack. Keep stacks short and focused:
 ```bash
 mergify stack new NAME       # Create a new stack/branch for new work
 mergify stack push           # Push and create/update PRs
+mergify stack sync           # Fetch trunk, remove merged commits, rebase
 mergify stack list           # Show commit <-> PR mapping for current stack
 mergify stack list --json    # Same, but machine-readable JSON output
 ```
 
+Use `mergify stack sync` to bring your stack up to date. It fetches the latest trunk, detects which PRs have been merged, removes those commits from your local branch, and rebases the remaining commits. Run this before starting new work on an existing stack.
+
 Use `mergify stack list` to see which commits have been pushed, which PRs they map to, and whether the stack is up to date with the remote. This is the go-to command to understand the current state of a stack. Use `--json` when you need to parse the output programmatically.
 
 ## CRITICAL: Check Branch Before ANY Commit

mergify_cli/stack/sync.py

@@ -128,14 +128,11 @@ async def get_sync_status(
 
     stack_prefix = f"{branch_prefix}/{dest_branch}" if branch_prefix else dest_branch
 
-    try:
-        base_commit_sha = await utils.git(
-            "merge-base",
-            "--fork-point",
-            f"{remote}/{base_branch}",
-        )
-    except utils.CommandError:
-        base_commit_sha = ""
+    base_commit_sha = await utils.git(
+        "merge-base",
+        "--fork-point",
+        f"{remote}/{base_branch}",
+    )
     if not base_commit_sha:
         console.print(
             f"Common commit between `{remote}/{base_branch}` and `{dest_branch}` branches not found",