fix(sdk): compute_auth_tag reads the owner secret from a file or stdin, never argv

[rmichelena]

Summary

crates/buzz-sdk/examples/compute_auth_tag.rs mints the NIP-OA auth tag an agent needs, and it took the owner's secret key as a positional argument. Anything on argv is readable through /proc/<pid>/cmdline by every local user for the life of the process, and the natural invocation (-- $(cat owner.key) <pubkey>) also routes the secret through a shell expansion. This runs on the owner's host — the one place the owner key must not leak.

This PR makes the secret arrive only through --owner-key-file <path> or --owner-key-stdin. The agent pubkey stays positional; --conditions becomes a flag.

• The old positional form is refused before anything else happens, without echoing its arguments, so a script still passing the secret on argv fails loudly instead of silently working.
• A bech32 nsec anywhere on argv is refused for the same reason. (A 64-hex secret and a 64-hex x-only pubkey have the same shape, so the old hex form is caught by argument count rather than by sniffing.)
• No error message includes what was read.

cargo run --release --example compute_auth_tag -- --owner-key-file owner.key <agent_pubkey_hex>
cargo run --release --example compute_auth_tag -- --owner-key-stdin <agent_pubkey_hex> < owner.key

Compatibility

Breaking for anyone invoking the example with the secret on argv — deliberately: that invocation is the leak. Nothing in the repository does (grep over *.md, *.sh, *.py, *.ts, *.yml; the benchmarks/.../test_keys.py comment only cites this file for its fixture values). Same tag output as before for the same inputs.

Test plan

• cargo test -p buzz-sdk --example compute_auth_tag — 5 passed: file form and stdin form produce a tag that verifies against the owner pubkey; the old form is refused and the refusal does not echo the secret; missing source / missing pubkey; empty source.
• cargo fmt -p buzz-sdk -- --check, cargo clippy -p buzz-sdk --example compute_auth_tag -- -D warnings clean.
• In production use on a closed relay since 2026-09-15 (owner-side tag minting for a headless agent), applied as a source patch on 3c7f288c; this is the same change rebased on main@6dfd145cd.

🤖 Generated with Claude Code

[github-actions]

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