fix(plugins): read the salt before minting one, and stop overclaiming backend support

Two findings from an independent review of this branch.

_install_salt went straight to create, fsync, link, unlink on every call. All but
the first process finds the salt already published, so each hook paid an fsync to
discover that, on a path documented as appending a line and returning. Hooks are
separate processes firing on every tool call inside a few-second budget.
Measured: cold process one fsync, warm process zero, same salt.

The deepseek README said the backend recognizes DEEPSEEK_HARNESS so usage
surfaces by name. It does not yet. That value, along with STRANDS, ZAPIER,
MEM0_PLUGIN, PI_AGENT and VERCEL_AI_SDK, buckets into OTHERS until
mem0ai/platform#3602 ships, so the README now states the dependency and links it.
The neighbouring comment in mem0-strands was already accurate and is unchanged:
it says recognized values live in the allowlist without claiming this one is in
it.

265 passed, 8 skipped.

Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb

integrations/agent-plugin-core/python/telemetry.py

@@ -133,6 +133,18 @@ def _install_salt() -> str:
         return _salt_cache
 
     path = _salt_path()
+    # Read before writing. Hooks are separate processes firing on every tool
+    # call, so all but the first find the salt already published; going straight
+    # to create-fsync-link-unlink meant every one of them paid an fsync to
+    # discover that, on a path whose whole promise is appending a line and
+    # returning.
+    try:
+        _salt_cache = path.read_text(encoding="utf-8").strip()
+        if _salt_cache:
+            return _salt_cache
+    except OSError:
+        pass
+
     temporary = path.with_name(f"{path.name}.{os.getpid()}.tmp")
     try:
         path.parent.mkdir(parents=True, exist_ok=True)