chore: gitignore agent session working files (LEO-345) (#25)

Extend the unpublished-planning ignore to the full set of agent session
working files (PLAN.md, TICKET.md, FINDINGS.md, COST.md) used by the
phase-handoff workflow. All of .claude/ stays ignored: this is a public
repo, so a committed settings.json would override the model choice of
anyone who clones it — the local model pin is instead a gitignored
symlink into the private settings share.

Co-authored-by: Agent for Leonard Janke <leonard.janke.agent@proton.me>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>