feat: add interactive contract playground script (#517)

[B3rnard601]

Adds scripts/playground.sh, an interactive CLI for exercising the most common contract calls without hand-writing cargo-contract commands:

1. Register Property (property-token :: register_property_with_token)
2. Create Escrow (escrow :: create_escrow_advanced)
3. Stake Tokens (staking :: stake)
4. Vote on Proposal (governance :: vote)
5. Create Insurance Policy (insurance :: create_policy)

• Resolves contract addresses from deployments//.json (written by scripts/deploy.sh), falling back to manual entry if a contract hasn't been deployed yet.
• Prints the full cargo-contract call output plus an isolated "Emitted events" section.
• Supports --help / -h with full usage instructions.
• Documented in docs/onboarding-checklist.md under Hands-On Exploration.

Follows the existing scripts/deploy.sh conventions for logging, network selection (NETWORK env var) and signer selection (SURI env var).

Closes #517

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[B3rnard601]

The failing checks (Rustfmt, Clippy, and the contracts/analytics/contracts/fractional build/test jobs) appear to be pre-existing issues on main unrelated to this PR — this change only touches scripts/playground.sh and docs/onboarding-checklist.md. The analytics Clippy failure, for example, is in contracts/analytics/src/lib.rs:272-274, a file this PR doesn't modify.