[wave 2] core: Safe/multisig + external-signer deploy path (no raw private key required)

[robercano]

What

Production teams deploy through a Safe or hardware wallet, not a raw DEPLOYER_PRIVATE_KEY env var. Add a signer abstraction so deploy/config transactions can be (a) signed by a pluggable external signer, or (b) emitted as Safe transaction proposals (batch) instead of being sent directly.

Scope sketch

• @redeploy/core: generalize jsonRpcProvider into a signer interface (private-key signer stays as one implementation); a "propose" execution mode that collects the ordered tx batch (to, data, value) instead of broadcasting.
• Safe integration: emit a Safe Transaction Builder-compatible JSON batch (minimal viable) and/or propose via Safe Transaction Service API.
• Interaction with resume semantics: proposed-but-not-executed txs must not be journaled as complete; document the confirm-then-resume flow.

Why

Removes the single biggest adoption blocker for mainnet use.