feat(acp): ACP registry readiness — agent auth, session/set_model, release archives

[raythurman2386]

Summary

Makes raven --acp a valid, complete ACP v1 agent so it can be listed in the
Zed ACP registry. The registry's --auth-check requires initialize to
return authMethods with at least one agent/terminal method — raven
returned [], which was the hard blocker.

• Advertise an agent auth method (agent-auth) in initialize. Raven
  authenticates with provider credentials already resolved in-process
  (env / config / .env), so authenticate is a no-op acknowledgement that
  validates the methodId and rejects unknown ones.
• Add session/set_model and advertise sessionCapabilities.set, matching
  the surface of grok-build / codex-acp.
• Standardize the invocation on raven --acp across docs and source
  comments (the registry launches cmd + args verbatim, so the entry uses
  args: ["--acp"]).
• Publish release archives (.tar.gz/.zip) with a stable
  raven/raven.exe and pinned sha256, for the registry's binary
  distribution. Raw binaries are kept for the installers.
• Enable macOS builds in the release matrix for full platform coverage.
• Add a 16×16 monochrome currentColor registry icon (assets/icon.svg).

Related issues

Prepares the repo for a Zed ACP registry submission (Phase C/D).

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets -- -D warnings
• cargo test (594 passed, incl. 5 new ACP tests)
• cargo build --release
• cargo check --target x86_64-pc-windows-gnu
• Live ACP handshake probe: initialize returns authMethods with
  agent-auth (type agent); authenticate accepts it and rejects
  unknown ids; session/set_model works.

Checklist

• Tests pass locally
• Code is formatted and lint-clean
• Docs updated (usage.md, troubleshooting.md, README, ROADMAP, CHANGELOG)
• CHANGELOG.md updated (if applicable)