[guide] ADR back-fill for knowledge system, skill registry, CEL triggers, and hub/spoke is on the roadmap but no ADRs exist for these subsystems

## Documentation Gap The public roadmap at `src/docs/roadmap.md` lists "ADR back-fill for remaining subsystems" as a **Next** item, naming four specific areas that lack Architecture Decision Records: - Knowledge system - Skill registry - CEL/channel triggers - Hub/spoke mechanics The `src/docs/adr/` directory currently has 17 ADRs (ADR-0001 through ADR-0017). None of the four subsystems above have corresponding ADR files. The knowledge system has design docs (`src/docs/design/knowledge-system.md`) but no ADR capturing the decision. The skill registry (`ADR-0012` is listed in the roadmap reference but not yet in the adr/ directory). ## Impact Contributors and operators working on or near these subsystems have no durable record of the architecture decisions behind them. This makes it harder to: - Understand why these subsystems work the way they do. - Know which constraints are intentional vs. historical accidents. - Review PRs that touch these areas with informed architectural context. ## Recommendation Create ADR stubs (even in Proposed status) for each of the four subsystems listed in the roadmap. Stubs can be short — just enough context and decision language to unblock review, with consequences marked "TBD" where not yet settled. The roadmap already links this work to tracking issue [#2811](https://github.com/kubestellar/hive/issues/2811). --- *Filed by guide agent (ACMM L4 — issues-only mode)* --- 🐝 **Hive Agent**: `guide` | **Instance:** `hosted-available-oke-11-placeholder-r05x` | **SHA:** `unknown` — hive: agent=guide backend=copilot model=claude-sonnet-4-6 copilot=1.0.78