Add four-language RunReceipt reader examples

[bigboateng]

Why

Yield now exposes strict yield.observation.v1 receipt readers in every
supported language. Users need runnable examples that show the shared portable
boundary without implying that an active skill can inspect its own receipt.

What changed

• add standalone RunReceipt readers for Go, TypeScript, Python, and Rust
• accept exact canonical receipt-object bytes and verify them before output
• emit the same allowlisted summary from every language
• add one shared harness for valid, trailing-data, digest-mismatch,
  unknown-field, partial-output, and private-value tests
• run the reader harness in the existing all-language examples CI job
• document the post-run reader boundary and link it from the examples and
  receipt references

The readers are not skill workflows and do not have skill.json files. They do
not read journals, resolve local receipt storage, project receipts, participate
in replay, or change foreground execution. Go remains the sole journal-to-
receipt projector.

Verification

• repository formatting, vet, tests, and build
• all 45 existing example workflows
• all four receipt readers and strict rejection/privacy cases
• Rust clippy with warnings denied
• release tests and release-control checks

No schema, CLI, SDK, Changeset, or generated skill-library files changed.