feat(hindsight): probe API for update_mode='append' support, dedupe across processes

Mirrors the pattern already shipping in hindsight-integrations/openclaw:
probe `<api_url>/version` once per process, gate on Hindsight ≥ 0.5.0.
When supported, retains use a stable session-scoped `document_id`
(`session_id`) plus `update_mode='append'` so cross-process retains for
the same session merge into one document instead of producing
N-different-process-stamped duplicates. When unsupported (or probe
fails), fall back to the existing per-process unique
`f"{session_id}-{start_ts}"` document_id with no `update_mode` — the
resume-overwrite fix (#6654) keeps working unchanged on legacy servers.

Closes the dedup half of #20115. The proposed `document_id_strategy`
config knob isn't needed: auto-detection via the same /version probe
the OpenClaw plugin already uses gives the same outcome with no extra
config burden, and the choice is purely a function of what the server
can do.

Plumbing
--------
- Module-level helpers (`_meets_minimum_version`, `_fetch_hindsight_api_version`,
  `_check_api_supports_update_mode_append`) cache the result per api_url
  so every provider in the process gets one /version round-trip.
- One-time WARN logged when the API is older than 0.5.0, telling the
  user to upgrade for cross-session deduplication.
- New instance helper `_resolve_retain_target(fallback_doc_id)` returns
  `(document_id, update_mode)` based on cached capability. Wired into
  `sync_turn` and the `on_session_switch` flush path.
- For local_embedded mode, the probe URL is taken from the running
  client (`client.url`) so we hit the actual daemon port rather than
  the configured default.
- `update_mode` is set on the per-item dict; `aretain_batch` already
  threads `item['update_mode']` into the API call.

Tests
-----
- `TestUpdateModeAppendCapability` (5 cases): legacy fallback, modern
  stable+append, per-url cache, one-time warn, flush-on-switch resolves
  against the OLD session.
- Existing `_make_hindsight_provider` factory in the manager-side test
  file extended to seed `_mode`/`_api_url`/`_api_key`/`_client` and stub
  `_resolve_retain_target` so the bypass-init pattern keeps working.

E2E verified against installed `~/.hermes/hermes-agent`:
- Legacy probe (unreachable host) → `legacy-session-<ts>` doc_id,
  no `update_mode`.
- Modern probe (live local_embedded 0.5.6 daemon) → stable
  `modern-session` doc_id + `update_mode='append'`.
- `test_hermes_embedded_smoke.py` passes (90s).

tests/agent/test_memory_session_switch.py

@@ -248,6 +248,14 @@ def _make_hindsight_provider():
     provider._atexit_registered = True
     provider._ensure_writer = lambda: None
     provider._register_atexit = lambda: None
+    # Mode + API state used by _resolve_retain_target; stub the resolver
+    # so tests don't actually probe the API. Real probe behavior is
+    # exercised by tests in tests/plugins/memory/test_hindsight_provider.py.
+    provider._mode = "cloud"
+    provider._api_url = ""
+    provider._api_key = ""
+    provider._client = None
+    provider._resolve_retain_target = lambda fb: (fb, None)
     # Stub the network-touching helper so any enqueued flush closure is
     # a no-op if ever drained in a unit test.
     provider._run_hindsight_operation = lambda _op: None