feat: bundle all three checkpoints in one repo, subfolder loading (v0.3.0)

convaiinnovations/laya now holds the whole family: English at the repo root,
multilingual and typed-decisions as subfolders. Agent and load() take a
subfolder argument, and snapshot_download is given allow_patterns so asking for
one checkpoint downloads only that one -- verified at 647 MB rather than the
full 2.4 GB.

Router defaults to the bundle; Router(standalone_repos=True) keeps the previous
one-repo-per-checkpoint layout, and those repos are unchanged.

RouteDecision['repo'] is now a readable 'repo/subfolder' string rather than the
internal spec tuple.

Tests: 88 routing (up from 77) covering both layouts, the repo-string helper and
local-path overrides, plus 29 criteria tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>