docs: define the M16 model & effort routing issue set

[danielPoloWork]

What

Adds .issues/M16-model-effort-routing-milestone.md — the canonical plan for milestone
M16 — model & effort routing,
following the M15 precedent (#233/#251).

Contents

• Theme: advisory-first model/effort routing — EADOS recommends (and, where the host allows,
  applies to delegated subagent work) the right model tier + effort per unit of work.
• Ratified decisions (2026-07-09): advisory-first / human keeps model authority (RFC-0001);
  tiers-not-names with a dated per-host catalog; OS-neutral effort scale low|medium|high|max;
  signals = tracker labels + sets-pattern / decision-heavy flags.
• Sequence: 16.1 [16.1] Routing policy as data + ADR: work signals → model tier & effort (tiers, not model names) #252 → 16.2 [16.2] tools/route_advice.py — pure evaluator: issue signals → recommended tier + effort + rationale #253 → 16.3 [16.3] Surface routing advice: Step-0 triage, /eados status, milestone-planning protocol (+ M15 backfill) #254 → 16.4 [16.4] Auto-apply routing where the host allows: delegated subagent work carries model + effort #255 (one PR each), with the Routing
  column dogfooded on the M16 items themselves.
• M15 backfill table: the maintainer-reviewed 2026-07-09 per-issue assessment in M16
  vocabulary — the data [16.3] Surface routing advice: Step-0 triage, /eados status, milestone-planning protocol (+ M15 backfill) #254 will apply to the 17 open M15 issues.
• Invariants: no top-level session model switching by the agent; no model names in policy; no
  new phase; agent drafts, human merges.

Verification

• eados_lint.py self-lint: OK (file staged before linting — gate-coverage reads git ls-files).
• Docs-only change; no code, templates, or specs touched.

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