chore: untrack agent session memory files from git

Agent history.md files and agent-memory/ directories are volatile
per-session logs managed by the clan tooling — not project source.
Remove tank/history.md from the index and add ignore rules so all
agent history files stay off git going forward:

  .clans/agents/*/history.md
  .claude/agent-memory/

The files remain on disk; only git tracking is dropped.

Author: cecilphillip