Add Spettro ACP agent

[cesp99]

As you guys suggested me on X i added Spettro to the registry!

[cesp99]

Summary

This PR adds Spettro to the registry — a terminal-first, multi-agent coding assistant written in Go, with configurable orchestrator/worker agent roles.

Spettro serves ACP over stdio via spettro --acp and supports the full session lifecycle (new/load/resume/list), image and embedded-context prompts, session config options (mode, model, permission, thinking level), slash commands advertised via available_commands_update, and permission requests mapped onto session/request_permission.

Entry details

• Distribution: binary tarballs for darwin-aarch64, darwin-x86_64, linux-aarch64, linux-x86_64, with pinned sha256 checksums from the release's checksums.txt (no Windows builds yet)
• Authentication: Terminal Auth (spettro-setup) — the client launches the binary without --acp, which opens Spettro's interactive TUI where the user configures provider API keys; ACP sessions reload the shared config each turn, so no restart is needed
• Icon: 16×16 monochrome SVG using currentColor

Validation

• uv run --with jsonschema .github/workflows/build_registry.py passes (schema, ID, version, URLs, icon; only the non-blocking missing-Windows warning)
• python3 .github/workflows/verify_agents.py --auth-check --agent spettro passes against the real v2.6.2 release binary: Auth OK: spettro-setup(terminal)