docs: document protocol trust boundaries

[Gaalbu]

Context

The code already enforces streamed writes, size checks and path sanitization, but the wire protocol and trust boundary needed a dedicated factual reference.

Changes

• Adds docs/protocol.md covering discovery, request, transfer, confirmation, errors and the current trust model.
• Links the protocol from the README.
• Changes Dependabot from monthly to weekly.

Validation

• Static review of server, client, session management and path-sanitization tests.
• Flutter commands were not executed because Flutter is unavailable in the audit environment.

Risks

Documentation/configuration only.

Follow-up

Design pairing and authenticated encryption using reviewed libraries; do not create a custom cryptographic protocol.

[Gaalbu]

Revisado e aprovado pelo proprietário. As alterações da auditoria foram validadas; autorizo o merge conforme o escopo documentado neste PR.