feat: scratchpad support for resumable analyses (#58)

[marknutter]

Summary

• Adds a scratchpad SQLite table (in the existing memory DB) with TTL-based auto-expiry for short-lived working memory
• New rlm/scratchpad.py module with save, get, list_entries, clear, delete, and promote operations
• CLI subcommands: rlm scratchpad save|list|get|clear|promote with configurable --ttl, --tags, --label, --session
• rlm scratchpad promote <id> graduates a scratchpad entry into long-term memory via the existing pipeline, merging tags and removing from scratchpad
• 5 new MCP tools: rlm_scratchpad_save, rlm_scratchpad_list, rlm_scratchpad_get, rlm_scratchpad_clear, rlm_scratchpad_promote
• Default TTL is 24 hours; configurable per-entry

Closes #58

Test plan

• uv run rlm scratchpad save "content" --label "test" --tags "foo" --ttl 2 creates an entry
• uv run rlm scratchpad list shows active entries with expiry time
• uv run rlm scratchpad get <id> returns full content
• uv run rlm scratchpad promote <id> --tags "extra" moves entry to long-term memory
• uv run rlm scratchpad clear / --expired removes entries
• All 131 existing tests pass

🤖 Generated with Claude Code

[marknutter]

Added 26 scratchpad tests in tests/test_scratchpad.py covering:

• DB CRUD (insert, get, delete, list with/without expired, clear all vs expired-only, tag round-trip)
• High-level API (save, get, list_entries, clear, promote to long-term memory, TTL defaults, session association)
• Promote behavior (tag merging without duplicates, custom summary, removes from scratchpad)
• Formatting helpers (format_entry_list, format_entry, EXPIRED display)

Total test count is now 157 (was 131). All passing. 🧱