feat(examples): add validated example catalog - #251

Summary

• add a generated, machine-readable index for all 111 real Cargo example targets
• attach source paths, summaries, exact commands, networks, environment variables, binaries, services, and hardware requirements
• validate catalog and README coverage in CI
• add agent and contributor guidance for finding and maintaining examples
• convert shared contract helpers from an accidental example executable into the package library
• make runtime artifact lookup independent of the caller working directory

Why

Cargo metadata exposed a helper module as a runnable example, while the human README omitted a real receipt-decoding target. Agents and users also had no structured way to distinguish offline examples from examples requiring networks, processes, credentials, or hardware.

Impact

Every real example now has a stable discovery record and a CI-enforced README entry. Consumers can select compatible examples without trial-and-error, and shared helpers no longer pollute the executable target list.

This PR is stacked on #250 so cataloged runtime requirements match the explicit endpoint configuration introduced there.