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/run.ts

@@ -27,18 +27,19 @@ import { DEFAULT_TRIALS, REPORTING_FLOOR, runCells, type RunCellsResult } from "
 import { resolveBenchAccess } from "./seed.js";
 import { detectSelfReviewSupport } from "./self-review.js";
 import { sdkAgentDriver } from "./sdk-driver.js";
-import { createSampleStore } from "./store.js";
+import { createSampleStore, DEFAULT_STORE_ROOT } from "./store.js";
 import { buildScoredSuite } from "./task-suite.js";
 
+// Re-exported so this command's existing importers keep resolving it from here;
+// its single source of truth is now the store, which owns the default root.
+export { DEFAULT_STORE_ROOT };
+
 /** The default turn cap a run is bounded by when not overridden. */
 export const DEFAULT_TURN_CAP = 40;
 
 /** The default wall-clock backstop (ms) a run is bounded by when not overridden. */
 export const DEFAULT_WALL_CLOCK_MS = 300_000;
 
-/** Where accumulated samples are stored when `--store` is not given. */
-export const DEFAULT_STORE_ROOT = "bench/results";
-
 /** Environment variable naming the tea login the benchmark authenticates through. */
 export const LOGIN_ENV = "GITEA_AXI_BENCH_LOGIN";