v0.2.1 — the onboarding release

The onboarding release. v0.2.0 shipped a key-free teammate path that could not
actually be reached by a teammate without a key — anyone installing @latest
before this tag gets that build, so this is the version to point a new coworker at.

• The key-free teammate path actually works now — found by dry-running the tool
  as a new coworker (clean GOBIN, a separate HOME per machine, a stand-in
  git remote, nothing from the checkout on PATH). Three defects, all inside the
  first four minutes:

  • ettle mcp required an API key to start, which made the key-free path
    unreachable by exactly the person it was built for. Client-side distillation
    exists so a teammate never needs a key; ettle_emit with atoms and
    ettle_respond make no model call at all. The key is now optional: without
    one the server serves that half and the model-calling tools return
    mcpserver.ErrNoKey, which names the alternative (ettle_distill + atoms)
    rather than reporting a bare missing key.
  • ettle mcp had no --room and kept the horizon in a process-local map,
    so two people on two machines never saw each other's atoms over MCP — the
    distributed path and the key-free path did not intersect anywhere. The
    horizon now rides the same transport.Transport seam the CLI uses
    (ettle mcp --room <name>, or --transport), with a fold-by-participant at
    the read side so a re-emit overwrites on an append-only bus. Default is
    unchanged: in-process, this server only.
  • ettle room join printed the one command guaranteed to fail for a
    keyless joiner (use it: ettle standup --room …). It now branches on whether
    a key is present and leads with what will actually work. ettle room with no
    args gained a real usage block.

  Verified end to end after the fix: a keyless "Bob" drove ettle mcp --room,
  emitted client-distilled atoms into a git-backed room, and the key-holding
  "Alice" reconcile caught the resulting collision (5/5 samples) across the bus.

• README install path made self-consistent, and the forge metadata refreshed.
  The quickstart now leads with go install …@latest, states the substitution in
  the direction the examples are actually written (ettle for go run ./cmd/ettle, not the reverse), and shows the installed-binary form in the
  ettle room block — the distributed flow needs no bundled testdata/, so that
  is the block a teammate actually copies. claude mcp add is given both ways.
  GitHub description, topics, and homepage updated: the old description predated
  rooms and the key-free path, and homepage was unset.