Feature request: add coordinator.domains: frontmatter to engineering/* SKILL.md files (for spec-coordinator tag-routing)

## Context

A separate skills framework — [Agent Upskill's spec-coordinator](https://github.com/agentupskill/skills-core/tree/main/spec-coordinator) — uses an SME registry to route cross-domain spec requests to relevant skills. The registry has two routing layers:

1. **Tag-match (deterministic)** — picks skills whose `coordinator.domains: [...]` frontmatter overlaps query tokens (confidence 1.0).
2. **Sonnet classifier (fallback)** — scores skills against the query (threshold 0.6 for picks).

The 10 `engineering/*` skills in this plugin are frequent SMEs on cross-domain specs (any query mentioning system design, testing strategy, ADRs, code review, deployment, etc.). Today they ship without a `coordinator:` overlay so spec-coordinator picks them classifier-only.

This works (the classifier consistently picks `engineering:system-design` on architecture queries, `engineering:testing-strategy` on test-strategy queries, etc. — see [agentupskill/skills-core#56](https://github.com/agentupskill/skills-core/pull/56) for the registry-discovery clarification ensuring classifier coverage on plugin-shape skills). But adding tag-match would:

- Reduce ~10 Sonnet classifier calls per spec query when token-overlap is sufficient.
- Eliminate classifier-confidence variance (±0.1 per `_threshold-calibration.md` F4.5) on queries with unambiguous match.
- Make routing more auditable (`method: tag` with a domain-list vs `method: classifier` with a confidence float).

`engineering:testing-strategy` is also Agent Upskill's seed `default_broadcast_skills` member (per [_routing-priors.md §Default-broadcast SMEs](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_routing-priors.md)) — it's auto-included on every spec-shape query regardless of classifier score. The `default_broadcast_skills` mechanism is registry-side (a list in spec-coordinator's routing-priors), so this issue doesn't affect that path; it's purely about reducing classifier overhead on the other 9 skills + sharpening tag-match for testing-strategy when its triggers ARE in the query.

## Specific request

For each of the 10 `engineering/*` skills, add a `coordinator.domains: [...]` block to the `SKILL.md` frontmatter. Suggested domains per skill (calibrated against Agent Upskill's [_routing-priors.md](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_routing-priors.md) Z4/Z7/Z8/Z9/Z10/Z17 ambiguity zones):

| Skill | Suggested `coordinator.domains` |
|---|---|
| `engineering/architecture` | `adr`, `architecture-decision`, `decision-record`, `trade-offs`, `consequences` |
| `engineering/code-review` | `code-review`, `pull-request-review`, `security-review`, `pr-review`, `n+1-query`, `injection-risk` |
| `engineering/debug` | `debug`, `bug`, `reproduce`, `isolate`, `diagnose`, `stack-trace`, `error-message` |
| `engineering/deploy-checklist` | `deploy`, `deploy-checklist`, `pre-deploy`, `release`, `rollback`, `migration-gate`, `feature-flag` |
| `engineering/documentation` | `documentation`, `readme`, `runbook`, `onboarding-guide`, `api-docs`, `architecture-docs` |
| `engineering/incident-response` | `incident`, `incident-response`, `outage`, `postmortem`, `triage`, `severity`, `oncall` |
| `engineering/standup` | `standup`, `daily-standup`, `yesterday-today-blockers`, `weekly-update` |
| `engineering/system-design` | `system-design`, `architect`, `service-boundary`, `data-modeling`, `api-contract`, `concurrent-writer`, `multi-tenant` |
| `engineering/tech-debt` | `tech-debt`, `technical-debt`, `code-health`, `refactor`, `maintenance-backlog` |
| `engineering/testing-strategy` | `test-strategy`, `test-plan`, `testing`, `eval-methodology`, `tier-1`, `tier-2`, `coverage` |

The `coordinator:` block is namespaced under a custom-coordinator key so it doesn't conflict with existing Anthropic-defined frontmatter. Schema documented in [`_routing-algorithm.md` §Inputs](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_routing-algorithm.md).

## Why upstream rather than a downstream patch

Plugin frontmatter ships with the plugin distribution and is not editable downstream. Without the overlay shipped here, every spec-coordinator installation falls back to classifier-only routing for all 10 `engineering:*` skills.

## Empirical context

PR-#320 ([Boogles-Playhouse/claude-skills#320](https://github.com/Boogles-Playhouse/claude-skills/pull/320)) drafted an 8-PR substrate-epic spec that needed `engineering:system-design` + `engineering:testing-strategy` in the 6-SME panel. The classifier sweep correctly picked them post-fix (see [agentupskill/skills-core#56](https://github.com/agentupskill/skills-core/pull/56)), but tag-match would have been faster + more deterministic. F-CAL-6 in [_threshold-calibration.md](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_threshold-calibration.md) records the calibration trace.

## Acceptance

- Each `engineering/*/SKILL.md` gets a `coordinator.domains: [...]` block.
- A short note added to the plugin's README explaining the field's consumer (Agent Upskill spec-coordinator) so future skill authors know to populate it.
- No breaking changes — the field is additive.

Happy to send a PR if Anthropic prefers that route.

## Cross-references

- Routing algorithm: [_routing-algorithm.md](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_routing-algorithm.md)
- Ambiguity zones (engineering-touching): Z4 (HITL × system-design), Z7 (system-design × architecture), Z8 (incident-response × debug), Z9 (code-review × workers-best-practices), Z10 (testing-strategy × HITL), Z17 (substrate-epic with system-design + testing-strategy + claude-api + cloudflare:*) — all in [_routing-priors.md](https://github.com/agentupskill/skills-core/blob/main/spec-coordinator/references/_routing-priors.md)
- Companion issues (other plugin sources): [cloudflare/skills#49](https://github.com/cloudflare/skills/issues/49), [anthropics/skills#1091](https://github.com/anthropics/skills/issues/1091)