feat(examples): S3, Redis, Postgres SessionStore reference adapters (#842)

Stacked on #837. (Replaces #841, which was merged into #837's branch by
mistake — this re-splits it.)

Three copy-in reference `SessionStore` adapters under
`examples/session_stores/`:

- **S3** (`s3_session_store.py`) — JSONL part files per append; mirrors
`@anthropic-ai/claude-agent-sdk-s3` (TS)
- **Redis** (`redis_session_store.py`) — RPUSH/LRANGE lists + zset
session index; mirrors the TS Redis reference
- **Postgres** (`postgres_session_store.py`) — `asyncpg` + jsonb rows
with bigserial seq; new (no TS equivalent)

Not shipped in the wheel — these live in `examples/`, users copy the
file they need into their project. The sdist does include `examples/` so
the references travel with source releases. Backend deps are in the
optional `[examples]` group only.

**Tests** — 678 passed, 3 skipped (env-gated live suites)
- All test modules `importorskip` on their backend client so default CI
is unaffected.
- S3 unit tests use `moto` in-process; Redis uses `fakeredis`; both pass
the full conformance suite.
- Live e2e modules for all three are env-gated (`SESSION_STORE_S3_*`,
`SESSION_STORE_REDIS_URL`, `SESSION_STORE_POSTGRES_URL`) and skip
otherwise.
- Verified locally against moto, fakeredis, and dockerized
`postgres:16-alpine`.

Zero secrets / internal refs in the diff (scanned).

**Review follow-ups** (9 commits total)
- Docs: Production checklist, Retention → `delete_session_via_store`,
scrubbed internal TS path refs, S3 ref now points at the public TS
example path, README notes Redis key separator + Postgres schema
divergence.
- Build: sdist manifest now includes `examples/`.
- Tests: Redis + Postgres conformance tests swapped `isinstance(store,
SessionStore)` → `_store_implements()` after #837 dropped
`@runtime_checkable` from the base Protocol.

Author: qing-ant

.gitignore

@@ -28,6 +28,10 @@ env/
 .venv
 uv.lock
 
+# Local env files (live-backend test credentials etc.)
+.env
+.env.*
+
 # IDEs
 .vscode/
 .idea/