T03: Data model, JSON store, and session lock

[NagisaVon]

Summary

• define Pydantic v2 persistent and contract-facing models
• persist branch-keyed sessions under the common Git directory
• add fail-fast advisory session locks with PID and age stale detection
• export and snapshot-test status, batch assignment, and publish JSON Schemas
• cover round trips, schema versions, linked worktrees, concurrent writers, stale locks, and read-only access

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv run pytest

All local checks pass. No T03 task-spec deviations.