feat(gateway): repo-targeted Charter brief tool for multi-repo agents

[stackbilt-admin]

Context

charter#116 defines the integration goal: expose repo-targeted Charter brief retrieval through stackbilt-mcp-gateway for multi-repo agent workflows.

Charter-side charter_brief exists in per-repo charter serve, but gateway execution ownership belongs here.

Goal

Add a gateway tool that returns a Charter brief for an explicit target repo under tenant auth constraints.

Proposed contract (v1)

Tool: stackbilt_repo_brief

Input:

• repo: string (for example Stackbilt-dev/charter)
• optional freshness controls (refresh, ttlMinutes) only if needed after initial rollout

Output:

• canonical brief payload from Charter surface
• source metadata (repo, generatedAt, freshness hints)

Scope

1. Repo targeting and tenant-to-repo authorization using existing gateway OAuth model.
2. Brief retrieval strategy that avoids unnecessary cold start costs.
3. Contract tests for:
   • authorized repo access
   • unauthorized repo rejection
   • missing repo / missing Charter context behavior
   • deterministic payload shape

Dependencies / links

• Charter contract tracker: feat(gateway): expose charter_brief through stackbilt-mcp-gateway with per-tenant repo targeting charter#116
• Related Charter package-context proposal: feat: package ecosystem — configurable dependency orchestration via charter config charter#90

Notes

Keep this issue as execution source-of-truth in gateway repo. Charter #116 remains the consumer-side contract and cross-repo coordination issue.