fix(addie): make agent registration self-explanatory

[EmmaLouise2018]

Summary

Make member agent registration self-explanatory.

Four issues with the existing flow surfaced from a real user attempt:

1. The dashboard CTA was unclear. + Add agent did not match the registry concept of enrolled agents.
2. The Addie seed prompt was nonsensical for first-time registration. Clicking the button sent I want to add an agent for compliance monitoring. to Addie — compliance monitoring is a downstream benefit, not the reason to register.
3. Addie did not drive a complete intake. When asked to register, Addie listed only 2 of the supported auth modes and re-used the "for compliance monitoring" framing from the tool description.
4. No member-facing docs explained the click path. docs/registry/registering-an-agent.mdx did not show a member how to actually get from the dashboard to a registered agent.

This PR makes the path obvious end-to-end:

• Button: rename + Add agent → + Register agent on /dashboard/agents (header + empty state).
• Seed prompt: all three call sites (dashboard-agents.html x2, services/org-health.ts) now send Help me register my agent. — natural first-person phrasing instead of compliance jargon. Centralized as a REGISTER_AGENT_SEED_PROMPT constant in dashboard-agents.html, mirrored in org-health.ts.
• Addie tool description: rewrite save_agent to lead with AAO registry registration, list the four persistable auth modes (none / static bearer / static basic / OAuth client credentials), and explicitly state that agent type is server-resolved from a capability snapshot — never asked. Trimmed by ~45% by pushing the intake script into behaviors.md.
• Addie behaviors: new section in behaviors.md walks Addie through a structured intake (URL → auth method choice → matching fields → protocol) before calling save_agent. Includes a paste-it-all shortcut for power users, an explicit override of the "act immediately on a pasted URL" rule when intent is registration, a no-echo-secrets rule, and an error-recovery branch (probe timeout, auth rejected, validation, permission denied).
• OAuth user authorization is now described as a separate step — register with None, then click Authorize on the agent card. save_agent no longer advertises a fifth auth mode it can't actually persist.
• Docs: expand the "How agents end up in the registry" section in docs/registry/registering-an-agent.mdx with the explicit click path: sign in → /dashboard/agents → + Register agent → what Addie will ask. Adopts the post-feat(registry): remove discovered agents from public surface #3780 single-enrollment-path framing (no source: registered/discovered, no four-paths model — those are gone from the public registry surface). Aligns tier wording with the live gate (Professional tier or higher) and describes type resolution as a capability-snapshot read, not a live probe.
• Link fix: /login was 404ing; replaced with /auth/login (AuthKit handles both sign-in and sign-up via that route). Caught by check:owned-links in CI.

Rebase notes

Rebased on main after #3780 merged. The four-paths / source: registered|discovered framing this PR originally extended is gone from the public registry surface, so the docs change now adapts the click-path walkthrough into main's single-enrollment-path structure rather than restructuring around a "section 1" that no longer exists.

Test plan

• On /dashboard/agents, the header CTA reads + Register agent and the empty-state CTA reads Register your first agent.
• Clicking either CTA opens /chat with Addie pre-loaded with Help me register my agent..
• Addie's first reply mentions the AAO registry (not "compliance monitoring") and lists the four persistable auth modes.
• Addie does not ask the user for the agent type.
• Addie collects URL, optional name, auth method, matching credential fields, then calls save_agent.
• On org-health quick actions, the "Register a buyer/seller agent" link uses the same updated seed prompt.
• Mintlify docs render: ## How agents end up in the registry contains the numbered click-path; no stale source: registered/discovered or four-paths language remains.
• All /auth/login links in the docs and Addie behaviors resolve (no 404).