feat: prescribe sub-agent delegation and model tiering across the workflow lifecycle

[aagnone3]

Summary

Augments the eng workflow so the session's default agent is the orchestrator and validator across all six lifecycle stages — never the worker — delegating stage work to focused sub-agents and choosing each sub-agent's model by that unit's complexity.

• New canonical reference wf-development/references/subagent-delegation.md: orchestrator/sub-agent roles, when to delegate, a lifecycle delegation map (grooming → development → testing → review → delivery → documentation), a three-tier model-selection matrix (economy / standard / strongest, judged on exit-check ambiguity, reasoning depth, and blast radius), dispatch-brief contract, and verification rules (bounded retries, tier escalation on dry attempts, never validate with a weaker model than produced the work).
• Six lifecycle routers (wf-grooming, wf-development, wf-testing, wf-review, wf-delivery, wf-documentation) each gain a self-contained, stage-tailored "Sub-agent delegation" section, keeping routers valid for skills-only installs. wf-setup deliberately excluded (not a task-lifecycle stage).
• workflows-work.md: Orchestrated execution is now the default execution model (Inline demoted to fallback for trivial edits or hosts without a sub-agent mechanism); the fixed "always Opus for implementation subagents" tiering is replaced with complexity-based per-dispatch selection.
• workflows-orchestrate.md: Execute section opens with the coordinator-not-worker posture and adds an explicit dispatch step.
• WORKFLOW_SKILLS.md: documents the delegation model as a cross-cutting architectural layer.

Sub-agents never mutate shared tracker/board/PR state; the orchestrator owns every write, all verification, and keeps the session's own model for triage.

Testing

• bun test: 426 pass, 0 fail (architecture guardrails, link checks, vendored-script sync, counts all green)
• No agent/skill counts changed; no version or changelog hand-edits

Post-Deploy Monitoring & Validation

• No additional operational monitoring required: markdown-only workflow-policy change to plugin skills; behavior is exercised by consuming agents at session time and guarded by the architecture test suite.

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🤖 Generated with Claude Code

[aagnone3]

Pre-merge knowledge-disposition check (wf-documentation gate, against head 4e2e4f18)

Disposition: no separate compounding artifact required.

Rationale: the durable knowledge this work produced is the deliverable itself — the sub-agent delegation policy (skills/wf-development/references/subagent-delegation.md) and the delegation-model section of WORKFLOW_SKILLS.md. The two session learnings that emerged during review were folded back into the shipped policy rather than a docs/solutions/ entry:

1. Safeguards must travel with the dispatch (the no-recursive-delegation rule was threaded into the dispatch contract and brief template after review found it stranded in prose).
2. Host model inheritance makes omission the most expensive choice (explicit downshifted selection is now mandatory at every dispatch).

Out-of-scope residue was tracked as #260. No generated docs regions were touched; component counts unchanged.