Track in-repo runtimes (e.g. AH Westend) via tag-based workflow

Problem

Some runtimes like Asset Hub Westend live directly in polkadot-sdk and don't consume SDK crates via crates.io. The current tracker pipeline relies on detecting crate version changes in downstream Cargo.lock files, so these runtimes are invisible to it.

Proposed solution

Add a dedicated GitHub Actions workflow in polkadot-sdk that pushes a tag whenever an in-repo runtime's spec version changes (e.g. on merge to master). The tracker can then:

1. Detect the new tag
2. Diff commits between the previous and current tag
3. Extract PR numbers from the commit history
4. Annotate those PRs with the runtime's deployment status

This mirrors how the Discover step already works for release tags, but scoped to a single runtime's spec version changes rather than crate publishes.

Runtimes affected

• Asset Hub Westend
• Any future runtimes defined in-repo that bypass crates.io

Open questions

• Tag naming convention (e.g. westend-ah-v1005000)?
• Should the workflow live in polkadot-sdk or in deployment-tracker?
• How to handle the on-chain step for these runtimes (same RPC-based approach should work)

Start by reading the existing Discover step for release tags and the tracker flow that detects tags, diffs commits, extracts PR numbers, and performs the RPC-based on-chain step. Define the tag naming and workflow ownership for in-repo runtimes such as Asset Hub Westend, then verify that a spec-version change produces a tag the tracker can consume and annotate.