docs(problems): add cross-repo work decomposition problem doc

[guyoron1]

What

Adds docs/problems/cross-repo-work-decomposition.md — a problem doc on how a single unit of intent is split across repositories, who owns the parts, and what makes the parts one thing rather than several unrelated issues.

Why

Cross-repo coordination is referenced as an open question across architecture.md, code-review.md, and codebase-context.md, but has never been written up as one coherent problem.

The doc starts from what already exists rather than claiming a blank slate:

• Prerequisite issue creation — triage can file blocking issues in other repos, allowlisted via create_issues.allow_targets
• Independent per-repo discovery — each repo polls with its own filter and no shared state

It then names the four capabilities those primitives do not cover: decomposition, ownership, aggregation, and ordering/atomicity.

Structure

• Four shapes of cross-repo work (prerequisite chain, parallel slices, atomic contract change, mechanical fan-out) — they do not all need the same solution
• Three approaches, from "do nothing beyond what exists" through parent-issue coordination to a fullsend-owned work object
• A worked example from an external Jira-driven orchestrator, included for its visible trade-offs — repository grounding and sizing discipline transfer; its central state store is the model this project has deliberately avoided
• An explicit section on the tension with the repo as coordinator, since a cross-repo change is precisely the case where no single repo can see the whole change

Problem doc only — it explores and does not decide. If the parent-issue approach turns out to be sufficient for the atomic contract change, the principle survives intact and this becomes a linking convention rather than an architecture change.

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