Ref writers, client side: keys, namespaces and writers from caos-cli

[nishu-builder]

The client half of design/ref-writers.md: caos-cli can hold a ref writer key and use it. Part 3 of 4, stacked on #303.

• caos-cli ref-writer-key new|show makes a key. It lives in the checkout's git config as caos.ref-writer-key.
• caos-cli namespace new [<label>], writers list|add|remove <namespace> [<key> [<label>]] and ref-push <rev> <ref>.
• With a key set, caos-cli signs every push it makes, and signs each request with X-Caos-Write (caos::set_request_signer), so the jobs it starts can be granted its namespaces.
• dev/cli-test/worker gives each client test a fresh key. The new tests/ref-writers covers the hook end to end: content-named refs, founding a namespace, adding and removing a writer, and which jobs a run token lets write.

Conversations don't use any of this yet; #305 moves them in.

Verified here

• cargo fmt --check, clippy -D warnings and cargo test pass, except launcher::checkout_import_completes_partial_history_and_keeps_local_edits, which fails the same way on main in this container.
• Both lint scripts and nix build pass.
• I ran tests/ref-writers' client steps by hand against the built server and caos-cli: founding a namespace, the refusals, and writers add/remove.

Not verified here

• The caos-test suite, including tests/ref-writers' job steps: caosd up can't fetch flake inputs from inside its containers in this sandbox.

Stack

1. Propose ref writers: who may push a ref, and how jobs inherit it #298: design doc
2. Ref writers, server side: the pre-receive hook, run tokens, admission #303: server
3. this PR: caos-cli
4. Ref writers for conversations: namespaces, delegation, and closing the rest #305: conversations

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