feat: icm import — conversations from Claude, ChatGPT, Slack (#78)

* feat: add icm import command — import conversations from Claude, ChatGPT, Slack

New `icm import` command ingests conversation exports from multiple sources
and extracts structured facts into ICM memory with auto-classification
and entity detection.

Supported formats (auto-detected):
- Claude.ai JSON export
- ChatGPT conversations.json
- Claude Code JSONL sessions
- Slack JSON export
- Plain text / transcript files

New capabilities in extract.rs:
- classify_fact(): tags facts as kind:decision, kind:preference,
  kind:problem, or kind:milestone using keyword heuristics
- detect_entities(): identifies person/project names in conversational
  text using pattern matching (zero network, zero LLM)
- extract_and_classify(): composes extraction + classification + NER

Usage:
  icm import conversation.jsonl --project myapp
  icm import ~/chats/ --project myapp --dry-run
  icm import export.json --format chatgpt --project myapp

Inspired by MemPalace's convo_miner, adapted to ICM's architecture
(rule-based, zero deps, Rust).

* style: cargo fmt

* fix: clippy manual_try_fold in learn.rs

Author: pszymkowiak