feat(plugin): holistic import of on-disk Claude state into mem0

When a user installs mem0-plugin, mem0 starts empty even though Claude
Code has been quietly accumulating their CLAUDE.md instructions,
auto-memory, and subagent learnings on disk for months. The existing
hooks capture live state going forward but cannot backfill what already
exists.

This commit adds a one-shot importer that walks every Claude state
surface and backfills it into mem0:

- `mem0-plugin/scripts/import_claude_state.py` (new, ~575 lines):
  discovers CLAUDE.md hierarchy (+ @imports), .claude/rules,
  ~/.claude/projects/*/memory, and ~/.claude/agent-memory; chunks each
  file at markdown headings (H2 default, H3 for oversize sections,
  small-sibling merge, code-fence safe); tags chunks with metadata
  matching the existing mem0-mcp skill vocabulary (convention /
  user_preference / task_learning / anti_pattern / decision); POSTs
  sequentially to /v1/memories/ with infer=True (server-side dedup).
  Marker file at ~/.mem0/imports/claude-state.json tracks content-hash
  per chunk for idempotent re-runs and crash-resume safety.

  Flags: --dry-run, --reset, --no-infer, --source <type>.

- `mem0-plugin/scripts/on_session_start.sh`: adds a one-time nudge
  block after the existing bootstrap output. Emits the holistic-import
  prompt only when ~/.mem0/imports/claude-state.json doesn't exist
  and the SOURCE isn't "compact". Propagates to Cursor and Codex for
  free (shared bash entry point).

- README, CHANGELOG, plugin.json bumped to 0.2.0.

Patterned exactly on the existing on_pre_compact.py: stdlib only
(urllib, hashlib, pathlib, argparse), reuses _identity.resolve_user_id,
same logging setup, same ~/.mem0/ storage convention. Zero new packages,
zero new deps.

Smoke-tested locally: --help renders, missing MEM0_API_KEY exits 1
with helpful message, hook emits nudge when marker absent and stays
silent when marker present.

Research and design context: https://www.notion.so/Claude-State-Holistic-Import-Research-Findings-366f22c70c9081d7abdcc6b94a80e8d9

mem0-plugin/scripts/on_session_start.sh

@@ -84,4 +84,23 @@ is being captured to mem0 in the background as `metadata.type=compact_summary`.
 EOF
 fi
 
+# ── Holistic import nudge ────────────────────────────────────────────────
+# One-time nudge to backfill existing CLAUDE.md / MEMORY.md / agent-memory
+# into mem0. Silent after the marker file appears (i.e., after first run).
+# Only emit on startup or resume -- during compact the user is mid-conversation.
+if [ "$SOURCE" != "compact" ] && [ ! -f "$HOME/.mem0/imports/claude-state.json" ]; then
+  cat <<EOF
+
+## Holistic import available
+
+On-disk Claude state (CLAUDE.md, ~/.claude/projects/*/memory, ~/.claude/agent-memory)
+has never been imported into mem0. To preview what would be imported, run:
+
+  python3 "$SCRIPT_DIR/import_claude_state.py" --dry-run
+
+Then drop \`--dry-run\` to import. This nudge disappears after the first
+successful run. Pass \`--reset\` to re-import from scratch.
+EOF
+fi
+
 exit 0