fix(mem0-agent): detached hooks lost their payload, so nothing was ever written

Observed live: real sessions flagged durable candidates (a preference, two
insights, a convention) but every session showed 'flush events: 0'. The manifest
backgrounded flush and assist-error with '( cmd &)', so the parent exited and
closed stdin before the child could read it -- the child then had no session_id,
fell back to 'no-session', and drained an empty buffer.

Now the CLI reads the payload itself and re-execs a detached worker with
start_new_session, so the hook still returns in ~0.27s. Verified end to end:
sent=1, session_state=created.

Also records a surface hint per event and shows it in 'sessions', since the
manifest pins MEM0_EDITOR to the same value on every client.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

integrations/mem0-agent/hooks/hooks.spec.yaml

@@ -95,7 +95,7 @@ hooks:
     matcher: "Bash"
     command: "${CLAUDE_PLUGIN_ROOT}/bin/mem0-agent assist-error"
     timeout: 5
-    background: true
+    background: false
     blocking: false
     local_only: false
     status_message: null
@@ -106,7 +106,7 @@ hooks:
     matcher: null
     command: "${CLAUDE_PLUGIN_ROOT}/bin/mem0-agent flush"
     timeout: 5
-    background: true
+    background: false
     blocking: false
     local_only: false
     status_message: null
@@ -117,7 +117,7 @@ hooks:
     matcher: null
     command: "${CLAUDE_PLUGIN_ROOT}/bin/mem0-agent flush --reason precompact"
     timeout: 5
-    background: true
+    background: false
     blocking: false
     local_only: false
     status_message: null
@@ -128,7 +128,7 @@ hooks:
     matcher: null
     command: "${CLAUDE_PLUGIN_ROOT}/bin/mem0-agent flush --reason end"
     timeout: 5
-    background: true
+    background: false
     blocking: false
     local_only: false
     status_message: null