fix(plugin): expire stale state captures + document plugin-update restart

Memory expiration + recency
  Hook-side captures (session_state in on_pre_compact.py and
  compact_summary in capture_compact_summary.py) now set
  expiration_date = today + 90 days. These types describe a single
  moment of project state and become stale clutter after a quarter.
  Durable types written by the agent (decision, anti_pattern,
  convention, user_preference, task_learning, environmental) stay
  unexpired by design -- a decision from a year ago is still a
  decision.

  Skill updated with two new sections:
    - "Expiration: high-churn vs durable" -- table mapping each
      metadata.type to whether it should carry an expiration_date.
    - "Recency filter on recall" -- agent should layer a
      created_at >= <90 days ago> filter on recall when the user
      asks about *current* state, and skip it when looking for
      durable facts.

Plugin-update / MCP reconnect
  When the plugin updates, the MCP server connection in the running
  client goes stale -- there's no in-process reconnection path from
  inside the plugin. Documenting the workaround is the realistic
  fix: README now has an "Updating the plugin" section explaining
  /restart (Claude Code), quit-and-relaunch (Cursor), restart
  (Codex). The auth header re-reads MEM0_API_KEY from the new
  shell, so users don't have to re-enter the key.

mem0-plugin/scripts/capture_compact_summary.py

@@ -22,6 +22,7 @@ import os
 import sys
 import urllib.error
 import urllib.request
+from datetime import date, timedelta
 
 sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
 from _identity import resolve_user_id
@@ -45,6 +46,8 @@ if os.environ.get("MEM0_DEBUG"):
 API_URL = "https://api.mem0.ai"
 MAX_TAIL_LINES = 2000
 MAX_SUMMARY_CHARS = 50000
+# Compact summaries describe a single session's state -- stale after a quarter.
+COMPACT_SUMMARY_EXPIRY_DAYS = 90
 
 
 def tail_lines(filepath: str, n: int) -> list[str]:
@@ -92,6 +95,7 @@ def find_compact_summary(lines: list[str]) -> str:
 
 
 def store_summary(api_key: str, summary: str, user_id: str, session_id: str) -> bool:
+    expires = (date.today() + timedelta(days=COMPACT_SUMMARY_EXPIRY_DAYS)).isoformat()
     body = {
         "messages": [{"role": "user", "content": summary}],
         "user_id": user_id,
@@ -101,6 +105,7 @@ def store_summary(api_key: str, summary: str, user_id: str, session_id: str) ->
             "session_id": session_id,
         },
         "infer": False,
+        "expiration_date": expires,
     }
 
     data = json.dumps(body).encode("utf-8")