refactor(aura-cli): retire the dead-end runs list/rank CLI surface

Since cycle 0045 the flat runs.jsonl store has had no producer (sweep/mc/
walkforward persist to the family store; `aura run` does not persist), so
`aura runs list` / `runs rank` read a store nothing populates — a live dead-end
surface, the honesty gap the Runway milestone closes.

Resolve the C18 deferred fork (INDEX.md) as (b) RETIRE: drop the two CLI commands
+ their dispatch arms, usage fragments, and the now-unused load_runs_or_exit.
Families (sweep/mc/walkforward, C21) subsume standalone over-time comparison. The
aura-registry flat lib API (append/load/rank_by/optimize) is RETAINED — optimize
backs walk-forward's in-sample step, rank_by backs `runs family ... rank`.
`runs families` / `runs family` are untouched.

Decision recorded on #73 (user-directed (b), 2026-06-18). Cosmetic sub-item
(json! family-wrapper -> to_json() stdout key-order unification) deferred to a
follow-up: json! routes the embedded report through serde_json::Value, which
re-alphabetizes keys, so it needs manual JSON construction, not a trivial swap.

RED-first: the retire test asserted `runs list`/`rank` now exit 2 (was exit 0).
Verified: cargo build --workspace, clippy --all-targets -D warnings, and
cargo test --workspace all green.

closes #73

docs/design/INDEX.md

@@ -740,10 +740,12 @@ never a materialized-`Vec` scan at the call site. CLI surface: `aura mc`,
 `walkforward` / `mc` persist via `append_family` with an optional `--name`.
 Deferred (Non-goals): content-addressed identity + replay-dedup; the "run-diff"
 depth and ranking families against each other (cross-family, vs. within-family);
-and a live producer for the flat `runs.jsonl` standalone-run path — after this
-cycle no CLI command writes it (sweep/walkforward moved to the family store, and
-`aura run` does not persist), so `aura runs list` / `rank` await either a
-persisting `aura run` or retirement (forward-queue follow-up).
+and a live producer for the flat `runs.jsonl` standalone-run path — no CLI command
+writes it (sweep/walkforward persist to the family store; `aura run` does not
+persist). **Resolved (#73, 2026-06): retired** — `aura runs list` / `rank` dropped;
+families (C21) subsume standalone over-time comparison. The `aura-registry` flat
+lib API (append/load/rank_by/optimize) is retained — `optimize` backs
+walk-forward's in-sample step, `rank_by` backs `runs family … rank`.
 
 ### C19 — Bootstrap: blueprint → instance (recursive)
 **Guarantee.** Construction is a distinct phase, recursive at every level. Each