chore: stop tracking .pi/ — it blocks rebase for any session with local goal state #49

Problem

PR #43 committed the agent's local goal state into the repository:

.pi/.goals-pool-snapshot.json
.pi/goals/.goal-ledger-checkpoint.json
.pi/goals/goal_events.jsonl
.pi/goals/archived_goal_2026092111245901_mub1hdzf-5j5j22.md

.pi/ is per-session working state that the Pi agent writes to continuously. Tracking it has two consequences, one of which blocks work outright.

It breaks rebase and pull for any session that has local goal state. git rebase and git checkout refuse to proceed:

error: The following untracked working tree files would be overwritten by checkout:
	.pi/.goals-pool-snapshot.json
	.pi/goals/.goal-ledger-checkpoint.json
	.pi/goals/goal_events.jsonl
Please move or remove them before you switch branches.
Aborting

Hit while rebasing a feature branch onto main on 2026-09-22. The workaround is to move .pi/ aside and restore it afterwards, which is exactly the kind of step that turns into lost state when it is forgotten.

It also publishes local state, including goal text and ledger snapshots, to anyone with repository read access.

Suggested fix

1. Add .pi/ to .gitignore.
2. git rm -r --cached .pi so the files stay on disk but leave the index.
3. Decide separately whether the archived goal documents are wanted in the repository; if they are, they belong under docs/ or plans/, not under a path the agent overwrites.

Acceptance

• .pi/ is ignored and untracked.
• git rebase origin/main succeeds on a branch whose checkout does not change .pi/.
• No agent goal state is readable from the repository.