fix(plugin): deterministic mem0 user_id resolution

Hooks previously fell back to $USER when MEM0_USER_ID wasn't set,
producing a different user_id on every machine for the same person.
Result: a single account with memories scattered across many user
buckets, none of which can see each other.

Resolution priority (same in bash and python):
  1. MEM0_USER_ID env var (explicit override)
  2. ~/.mem0/identity.json cache (pinned to MEM0_API_KEY fingerprint)
  3. Derived: "mem0-" + sha256(MEM0_API_KEY)[:12]
  4. Fallback: $USER, else "default"

Same MEM0_API_KEY across machines now yields the same user_id without
the user having to set MEM0_USER_ID by hand on every laptop.

Resolver shipped as two tiny files instead of a shared module:
  _identity.sh -- sourced by bash hooks, exports MEM0_RESOLVED_USER_ID
  _identity.py -- imported by on_pre_compact.py, exposes resolve_user_id()

Hook integration:
  on_user_prompt.sh -- sources resolver, USER_ID interpolated into rubric
  on_session_start.sh -- emits an "Active user_id: <X>" header before the
    bootstrap text, so the agent's MCP search_memories/add_memory calls
    use the same bucket the hooks write to (closes the agent-side half
    of the symptom)
  on_pre_compact.py -- replaces inline env lookup with resolve_user_id()

Existing memories under previous $USER values are not auto-migrated.
The cache file regenerates on key rotation (fingerprint mismatch).

mem0-plugin/scripts/on_pre_compact.py

@@ -18,8 +18,11 @@ import json
 import logging
 import os
 import sys
-import urllib.request
 import urllib.error
+import urllib.request
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+from _identity import resolve_user_id
 
 log = logging.getLogger("mem0-capture")
 log.setLevel(logging.DEBUG)
@@ -219,7 +222,7 @@ def main():
         return
 
     session_id = hook_input.get("session_id", "")
-    user_id = os.environ.get("MEM0_USER_ID", os.environ.get("USER", "default"))
+    user_id = resolve_user_id()
 
     lines = tail_lines(transcript_path, MAX_TAIL_LINES)
     if not lines: