[workflows][release-tagging]create tags on version bumps, generate release notes

[quepcompute]

Description

Adds a lightweight workflow that fires a repository_dispatch event to
MystenLabs/sui-operations whenever a commit that changes Cargo.toml
lands on main. This is the trigger side of the automated Seal release
process — the actual tag creation and GitHub release notes are generated
by the receiving workflow in sui-operations.

How it works

• Workflow: .github/workflows/notify-release.yml
• Triggers on push to main with paths: Cargo.toml
• Fires repository_dispatch with event-type seal-version-bump to
  MystenLabs/sui-operations

Prerequisites

A secret SUI_OPS_DISPATCH_TOKEN must be set on this repo — a PAT
(or fine-grained token) with actions: write permission on
MystenLabs/sui-operations.

Test plan

• Confirm SUI_OPS_DISPATCH_TOKEN secret is present in the seal repo
• Merge a test Cargo.toml version bump and verify the workflow run appears
  and triggers the seal-version-bump dispatch in sui-operations