feat: add benchmark reporting command (task 0031)

Add the maintainer-facing `bench:report` command, the offline counterpart
to `bench:run`: it opens the sample store, drains it, aggregates against the
scored suite and bonus definitions, and prints the aggregator's comparison to
stdout. It renders whatever has accumulated, annotating incomplete coverage
rather than blocking on a complete matrix.

`parseReportArgs` is the pure argument seam (`--store`, `--help`, plus a
`--self-review` / `--no-self-review` variant selector). Unlike `bench:run` the
boundary is offline — it reads only the local store, no host or Agent SDK — so
the whole `runReportCommand` is deterministic and unit-tested, not smoke-run.

Move `DEFAULT_STORE_ROOT` to `store.ts` as the single source of truth,
re-exported from `run.ts` for its existing importers.

bench/store.ts

@@ -2,6 +2,9 @@ import { appendFileSync, mkdirSync, readdirSync, readFileSync } from "node:fs";
 import { dirname, join } from "node:path";
 import type { CellKey, ResultRecord } from "./result.js";
 
+/** Where accumulated samples are stored when no store root is given. */
+export const DEFAULT_STORE_ROOT = "bench/results";
+
 /** Run `read`, returning `fallback` when the target does not exist yet. */
 function ignoreEnoent<T>(read: () => T, fallback: T): T {
   try {