acp: advertise authMethods + handle authenticate

[iqdoctor]

What

• Advertise a minimal ACP authMethods entry from fast-agent ACP server (defaults to agent type per the RFD when no type is provided).
• Implement authenticate(methodId) handler (validates method id; returns empty AuthenticateResponse).
• Tighten ACP integration coverage (assert authMethods present; avoid flake by waiting for agent_message_chunk).
• Add internal notes for ACP registry + auth-methods (plan/acp-...).

Why

ACP supports authMethods in initialize and an authenticate RPC, but fast-agent currently returns authMethods=[] and does not implement authenticate. The draft RFD proposes typed auth methods; until SDKs expose those fields, this keeps everything within the current AuthMethod shape (id/name/description) and provides a stable hook for clients to trigger setup flows.

Notes

• Description uses a Markdown link for the config reference; happy to switch to plain URL if you prefer.
• The registry currently only accepts Agent Auth / Terminal Auth; this is a first step to make fast-agent a better ACP citizen.

[iqdoctor]

Note on the 2 new docs files in this PR:

• plan/acp-registry-auth-methods-plan.md: fast-agent specific phased implementation plan for ACP Registry + auth-methods RFD support (starting with the minimal agent auth method), including concrete code touch points and testing notes.
• plan/acp-auth-methods-survey.md: compact survey of the current ACP spec + registry requirements + SDK constraints (python-sdk) + a real reference implementation (codex-acp), to justify the minimal approach and document what can/can't be represented today.

If you'd prefer to keep PRs code-only, I'm fine dropping/moving these docs.

[evalstate]

Keep the plans in - at some point we should review them and turn them in to a code map for agents. Is there a good ACP Client to test this with?

[iqdoctor]

Client to test this with?

• zed.dev
• cosmos-vibe/Nexus-acp
• omercnet/vscode-acp

Also new ACP plugin for VS code coming soon:

• strato-space/acp-plugin [TODO]