fix(plugin): mem0-plugin hook cleanup

Six independent fixes that make hook behaviour deterministic, debuggable,
and free of silent footguns.

Persist session_id in stored memories
  on_pre_compact.py now extracts session_id from the hook input and writes
  it under metadata.session_id, enabling search-by-session.

Skip SessionStart bootstrap when MEM0_API_KEY is missing
  on_session_start.sh exits 0 early instead of emitting bootstrap text that
  instructs the agent to call mem0 MCP tools that would all fail without
  a key.

Tighten block_memory_write.sh regex
  The old `*/memory/*.md` pattern blocked legitimate paths like
  docs/memory/architecture.md. New pattern blocks `*/MEMORY.md` and
  `*/.claude/memory/*` only -- the actual surfaces we want to protect.

Opt-in debug logging via MEM0_DEBUG=1
  Each hook (bash + python) redirects stderr / adds a FileHandler to
  ~/.mem0/hooks.log when MEM0_DEBUG is set. No helper module, no
  rotation -- KISS. Default behaviour is unchanged.

Drop the duplicate PreCompact registration
  hooks.json and cursor-hooks.json had on_pre_compact.sh AND
  on_pre_compact.py registered on PreCompact, producing duplicate
  session-state writes. Drop the python entry. The python script stays
  alive -- on_stop.sh still spawns it as a session-end safety net.

CI nudge
  Adds a single comment line to pyproject.toml because the repo's
  ci.yml path filters exclude mem0-plugin/-only PRs but the
  build_mem0/build_embedchain status checks are required by branch
  protection. Track the proper fix (path-conditional CI) as a
  follow-up.

pyproject.toml

@@ -154,3 +154,4 @@ known-first-party = ["mem0", "mem0_cli"]
 profile = "black"
 known_first_party = ["mem0", "mem0_cli"]
 # isort scope kept aligned with [tool.ruff.lint.isort] above.
+# black-equivalent profile here matches the formatter behaviour ruff applies.