v0.5.0 — multi-repo sessions + prebuilt images

Two adoption features.

Multi-repo sessions (#20). tjor run --dir <path> (repeatable) mounts additional repositories into one session at their host paths, writable — so a single agent can work across several repos at once (a service and its client, a repo and a shared library). The primary workspace still anchors the session (id, identity, cwd unchanged); each --dir is verified to exist and be VM-shared before mounting.

Prebuilt, uid-agnostic agent images (#21). The agent image no longer bakes the host uid — the entrypoint aligns the agent user to the host uid at container start, so one image serves any user. This unblocks publishing: a release-tag CI workflow builds multi-arch (amd64+arm64) images and pushes them to GHCR (ghcr.io/bjorges/tjor-agent-<harness>), and an installed copy now pulls the image for its version on first run instead of building — the biggest first-run friction removed. Git checkouts and offline still build locally; [images] publish = false to always build.

Also: bind-mounted repos are trusted for git inside the cage (safe.directory = *). Full CHANGELOG in the repo.