project: add /api/v1/projects aggregation; sessions: add ?cwd= filter

- session.ListFilter gains Cwd *string; List appends working_dir = ?
  clause between Host and Since in fixed order (parameterised, column
  name is a literal)
- session.Handler reads ?cwd= and threads it to filter.Cwd; empty
  string treated as absent, consistent with other optional filters
- new package internal/domain/project:
  - Repository.List groups sessions by non-NULL working_dir in a single
    SQL round-trip; correlated subquery picks top_tool with tie-broken
    by tool ASC; Hosts/Tools deduped and sorted in Go after
    GROUP_CONCAT
  - Handler mounts GET /projects; resolveScope/clampLimit/clampOffset
    duplicated from session handler; ?owner= admin gating identical
- server.Server gains Projects *project.Handler inject field; mounted
  in /api/v1 Route block
- main.go and e2e test register projectRepo + projectHnd with steward