contrib: docs and examples for issues 293, 300, 281, 276

[Olorunfemi20]

Summary

Four contrib/examples entries, each scoped to one assigned issue. Per CONTRIBUTING.md and contrib/README.md, every change here is confined to contrib/examples/ — no files outside that folder are touched.

• contrib/examples/issue-293-policy-schema-migration-guide — a guide describing how to detect an outdated cached policy schema, a versioned migration helper patterned after the session store's storage-adapter approach (src/session.ts), and a test that runs the documented migration steps end to end against a fixture.
• contrib/examples/issue-300-deprecation-warning-pattern — a reusable deprecation pattern (JSDoc @deprecated plus a one-time console.warn) for an SDK method superseded by a newer client, including the exact CHANGELOG.md entry shape a maintainer would add. payments.ts and payments-client.ts don't currently have an actually-redundant method pair (payments.ts holds pure types/parsing that payments-client.ts's client builds on top of), so this documents the mechanism a maintainer can apply directly the moment a real deprecation is needed, rather than inventing a contrived one on a real export.
• contrib/examples/issue-281-release-process-guide — a stage-by-stage release process doc (versioning, changelog, build/verify, publish), noting what's automated vs. manual, linking to the actual .github/workflows/ci.yml, publish.yml, and verify-merged.yml. Written to be folded into CONTRIBUTING.md by a maintainer.
• contrib/examples/issue-276-rpc-timeout-budget — a timeout-budget wrapper for the policy-deployment RPC calls (simulate, deployInstance, recordDeployment), mirroring policy-client.ts's request-helper shape, with a distinct typed PolicyDeployTimeoutError (kept separate from PolicyApiError since a timeout means no server decision was ever received) and tests verifying the timeout fires per configured budget.

Each folder has its own README (or, for #281, RELEASE_PROCESS.md) explaining the requirement, the pattern, how it maps onto the real source file it targets, and how to run it.

Test plan

• npm run typecheck — passes
• npm test — 104 test files, 639 tests, all passing (includes the 24 new tests across the three code-bearing entries)
• npm run build — passes

closes #293
closes #300
closes #281
closes #276

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