Make Sieve the canonical forwarding format

Goal

Use the interoperable Sieve redirect/keep model for forwarding while preserving existing configuration compatibility.

Scope

• Compile forwards.json rules to deterministic Sieve scripts.
• Support full email addresses canonically and bare local-parts as legacy input.
• Explicitly define Junk forwarding policy.
• Add loop prevention and redirect limits per RFC 5228.
• Add validate/compile/test CLI commands.

Acceptance criteria

• Existing role forwards compile without behavior changes.
• keep + redirect semantics are explicit.
• Junk policy is configurable and documented.
• Loop and multi-hop tests pass.
• Live socials@ and hi@ delivery probes pass.

Implemented in commit 9f7fb90 and tracked in draft PR #8.

config/email.ts remains the source of truth; Buddy writes the compatibility JSON manifest and atomically compiles canonical RFC 5228 Sieve. Live end-to-end probes for socials@stacksjs.com and hi@stacksjs.com both delivered to chris@stacksjs.com, retained their source mailbox copy, and were cleaned up afterward.