Remove dead platform_type UI (superseded by specialisms in migration 409)

[bokelley]

Observed in #2808 DX review. Low priority refactor.

`dashboard-agents.html`'s connect form ships `platform_type` three separate times — once per auth-type field group (bearer/basic, OAuth auth-code, OAuth client-credentials). Each has a distinct `name=` suffix (`platform_type`, `platform_type_oauth`, `platform_type_cc`) so the active one's value is submitted.

The field is about the agent, not the auth method. Hoist it above the auth-type select so the operator fills it once, regardless of auth choice.

Proposal

1. Render one `platform_type` select above the auth-type select.
2. Remove the three duplicated selects from inside each field group.
3. Update the three save handlers to read the single hoisted field.

Scope notes

• Small (<30 lines of HTML/JS).
• No behavior change from the server's perspective.
• Cleanest way is to do it alongside a follow-up UI refactor.

Related: #2808.

[bokelley]

Re-scoping after digging in: platform_type is already dropped server-side (migration 409_drop_platform_type.sql, "@adcp/client 5.1.0 replaces the AAO's platform_type concept with [specialisms]"). The column doesn't exist on `agent_registry_metadata` anymore.

But the dashboard still renders three `` selects and the submit handlers still set `body.platform_type = platformType`. The server silently drops the field. It's dead UI sending dead data. Updated proposal: remove platform_type entirely from `server/public/dashboard-agents.html`: Three `` rows in `buildConnectForm` (bearer/basic, oauth, cc variants).

The `platformTypes` / `platformTypeOptions` arrays in `renderAgentsSection`.

`body.platform_type` on the save handlers (bearer/basic + oauth + implicitly the cc handler, which doesn't reference it but still renders the select).

The `platform_type` field in the `/connect` OpenAPI schema at `server/src/schemas/registry.ts`, if it was ever added there.

Small (~50-line delete). Agent specialism discovery replaces it.

Re-titling the issue accordingly.