docs: reframe trust model as run-your-own-node

[TaprootFreak]

Summary

Reframes the ## Trust model section in CONTRIBUTING.md so the run-your-own-node model is unmistakable.

The previous wording ("node is trusted, wallet is thin" / "the wallet trusts the node") was easily misread as trusting a third party. It now states the actual model: the Bitcoin full-node model — your wallet trusts your node, exactly as a Bitcoin wallet trusts your own bitcoind. "Trusted node" means your node.

Clarifications added:

• The node↔wallet split is packaging, not a trust boundary.
• Self-hosting gives trustlessness and privacy at once.
• A public operator can never steal / forge / double-spend (enforced cryptographically); using one is the same spectrum as an Electrum/SPV server vs. your own bitcoind.
• The thin wallet/SDK is not a compromise — trustlessness comes from running your own node, not from anti-node logic in the client.

The existing hard rules (thin client, self-hosting easy, thin SDK) are preserved.

This block is mirrored verbatim across zk-coins/node, /sdk, /app, /docs — companion PRs keep all four identical.

Notes

• Docs-only change, no code paths touched.
• Prettier-clean; block verified byte-identical across all four repos.