#3291 stagecraft: example tracker extension and the writing-a-tracker guide (#3274 step 3)

Part of swamp-club #3274 (let users build their own tracker as a separate extension). Design settled with Seth on 2026-10-12: Route B. stagecraft and a tracker share no code. A tracker extension implements primitive methods with fixed inputs and one result record each. stagecraft ships the publish and claim logic as workflows whose own steps hold the ledger, cursor and ticket index, and whose tracker steps call the bound tracker instance by name. Tested on a remote worker under swamp serve (see #3274): this works where runModel does not. A JSR package was ruled out.

This step (3 of 3)

Prove it and document it. Depends on step 2.

• A minimal example tracker extension in stagecraft/examples/ (not Jira), repo-only and never pushed. It is bound, validated, started against and published to, and passes the conformance workflow. Verification runs it through a checks.yaml entry.
• The writing-a-tracker guide, shipped in the package through additionalFiles: the primitive methods and their result schemas, capabilities, origin, credentials and redaction, idempotency keys, and running conformance.
• The skill's authoring reference gains an Another tracker section.
• Manifest bump.