Add SourceOS interaction implementation ledger

[mdheller]

Summary

Adds a compact implementation ledger for the completed SourceOS interaction substrate wave.

This PR adds:

• docs/contract-additions/sourceos-interaction-implementation-ledger.md

The ledger records:

• canonical sourceos-spec PRs and commits;
• downstream implementation PRs and commits;
• validation lanes used;
• authority boundaries;
• non-goals;
• remaining non-blocking future work.

Scope

Documentation only. No schema behavior changes.

Related

• Add SourceOS interaction substrate for Noetica and AgentTerm #106
• Index SourceOS interaction substrate in contract catalogs #115
• Generate SourceOSInteractionEvent types #117
• Add SourceOS interaction reference flow packet #118
• Index SourceOS interaction substrate at top level #119
• Emit SourceOSInteractionEvent from Noetica chat lifecycle SocioProphet/Noetica#16
• Vendor generated SourceOS interaction types SocioProphet/Noetica#17
• Add SourceOS contract sync check SocioProphet/Noetica#18
• Document SourceOS interaction reference flow pointer SocioProphet/Noetica#19
• Render SourceOSInteractionEvent governance traces agent-term#46
• Vendor generated SourceOS interaction Python types agent-term#47
• Add SourceOS contract sync check agent-term#48
• Document SourceOS interaction reference flow pointer agent-term#49
• Bind SourceOSInteractionEvent at Superconscious task boundary SocioProphet/superconscious#60
• Attach AgentPlane refs to SourceOSInteractionEvent evidence binding SocioProphet/agentplane#253
• Document SourceOS interaction reference flow pointer SocioProphet/agentplane#254