Doghouse: avoid repo-path collisions in JSONL snapshot storage

[flyingrobots]

Follow-up to #4.

Why

JSONLStorageAdapter._get_path() currently derives the storage directory with repo.replace("/", "_").
That aliases distinct repositories onto the same path. For example:

• a/b_c -> a_b_c
• a_b/c -> a_b_c

Doghouse is a flight recorder. Cross-repo history contamination is a trust failure.

Current behavior

File:

• src/doghouse/adapters/storage/jsonl_adapter.py

Today, repo identity is not injective once / is rewritten to _.
Different repos can append snapshots into the same JSONL ledger.

Desired outcome

Use a storage identity scheme that cannot alias valid repo names.

Acceptable approaches include:

• nested directories (owner/name/pr-123.jsonl)
• URL-safe encoding
• a stable hash with enough context to stay inspectable

Acceptance criteria

• Two distinct repos cannot resolve to the same storage path.
• Add a regression test proving a/b_c and a_b/c land in different paths.
• Existing read/write behavior remains simple and local-first.
• Any migration behavior for previously written histories is documented if needed.