Workspaces: references-git for read-only dependency sources

[bracyw]

Adds a references-git: map to workspace.yml for open-source dependency repos the agent reads for context but never works in.

• setup.sh checks each one out at references/<name>/: one commit deep, not on a branch, at its optional tag/branch pin or the default branch, with pushes disabled. Every setup run refreshes it; a failed fetch keeps the old copy. No repos/ store, no worktrees, and /references/ is excluded from git.
• new-worktree.sh refuses references. ci/check.sh validates entries and rejects a name used in both repos: and references-git:.
• The template's workspace.yml has a commented example, and its CLAUDE.md tells the agent to read references (in preference to web docs) and never edit them. README and design.md are updated, and the managed scripts are copied into all three workspaces.
• The -git suffix leaves room for other kinds of references later.
• argos-dev gets three references pinned to what Argos develop uses: socketioxide (socketioxide-v0.18.7), socket.io (socket.io-client@4.8.1, a monorepo with client, server and engine.io), and zenoh (1.10.1). About 55 MB in total. Pins may contain @.

Testing: tests/e2e.sh passes all 40 checks, 4 of them new; one pin uses an @-style tag (shallow, pinned, detached, push blocked, refreshed on re-run, new-worktree refusal, plus 2 check.sh cases). check.sh and shellcheck pass.