refactor(memory): S3-generic store on boto3; RustFS is the tested server (no MinIO naming)

The plane's memory store no longer names one vendor. `MinioStore`/`SdkClient`/
`minio_settings`/`MINIO_KEYS` become `S3Store`/`S3Client`/`s3_settings`/`S3_KEYS`;
the URL scheme is `s3://<bucket>[/<prefix>]` (`s3://` alone uses `S3_BUCKET`); the
settings are `S3_ENDPOINT` (a full URL), `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY`,
optional `AWS_SESSION_TOKEN` and `S3_REGION`. The client is boto3 (extra `s3`,
imported lazily): SigV4, path-style addressing, SDK checksum trailers off, paginated
listing, versions and delete markers listed and deleted by version (`purge`, used by
the contract tests' teardown so the bucket keeps nothing of a run).

Behaviour is unchanged: the store never configures its bucket and verifies
versioning, the `runs/` expiry rule, S3 Select and tag reads at startup; no local
fallback; every row carries every queryable field with "" placeholders. The admin
hints read `--endpoint-url "$S3_ENDPOINT"`. The fake client still reproduces RustFS's
two Select failures (unbound leading-row field, null-typed column).

Docs: rustfs.md stays the tested server's setup page (policy JSON unchanged, env
names updated, any S3-compatible store with versioning, lifecycle, tags and Select
works); memory.md, ops/ax/README.md, README.md, steering and the two current spec
designs use the S3 naming. Retired specs and committed measurements are untouched.

Real server (RustFS, OUSAST_MEMORY_TEST_S3=1): test_plane_memory + test_learn_compile
75 passed, 1 skipped. Store copy of the live local store to a temporary prefix: every
count matched (776 row objects, 2982 rows, 22 facts, 788 excerpts, 2601 responses,
2 programs, 1793 index entries), 4190 versions purged afterwards.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/learned-decision-engine/tasks.md

@@ -47,7 +47,7 @@ a model or embedding call.
       redacted, delta diff); `learn/examples.py` and `ousast learn memory build` (label + feature record +
       excerpt -> `example` row; refuses `origin: user` and sources outside `sources.toml`; exits 2 when more than
       10% of a source has no excerpt); `example` added to `KINDS`; `put_blob`/`get_blob` on both backends.
-    - Tests: `test_learn_excerpt.py`, example-builder refusals, blob round trip on FileStore (MinIO gated).
+    - Tests: `test_learn_excerpt.py`, example-builder refusals, blob round trip on FileStore (S3 gated).
     - _Requirements: 3.2, 3.6, 7.2_
   - [x] 6.2 Metered embeddings and the cache
     - `plane/models/openrouter-embedding.yaml` (`input_per_m: 0.02`), `MeteredEmbeddingClient` in