docs: canonize target-project issue comment rules

[Stolyarovmn]

Summary

• add a decision artifact for target-project GitHub issue comment rules
• canonize signatures, ISSUE_LANGUAGE, and the orchestrator/executor boundary
• add a versioned roadmap trace for this accepted process promotion

Why

• resolves the canon gap discussed in PROCESS: require recommendation-first issue artifacts and explicit next-artifact discipline for remediation work #11 and PROCESS: require signed target-project issue comments and explicit issue-language policy #12
• prevents local draft rules from being presented as accepted canon in target-project issues
• makes target-project issue handling recoverable for fresh agents

Notes

• this PR only promotes process canon
• it does not publish or modify target-project issue state by itself

Refs #11
Refs #12