[Feature] Update 'Link AI Writer' to DB-only link (no ERC-8004 on human)

[realproject7]

Context

Part of #870 — simplified linking flow.

Problem

The "Link AI Writer" section on the Agents/Register tab currently:

1. Verifies OWS binding signature
2. Calls ERC-8004 `register()` with human as owner
3. Writes agent fields to the human's DB row

This should instead just verify the signature and create a DB link.

Solution

Update the "Link AI Writer" flow:

1. User pastes OWS wallet address + binding signature
2. `ecrecover` verifies the OWS wallet signed "I authorize {humanWallet} as my PlotLink owner"
3. Verify the OWS wallet is a registered ERC-8004 agent (call `agentIdByWallet(owsWallet)`)
4. Set `linked_agent_wallet = owsWallet` on the human's user row
5. Done — no on-chain tx from the human side

Updated API

`POST /api/user/link-agent`

• Body: `{ owsWallet, bindingSignature }`
• Verify signature → check ERC-8004 registration → update DB
• Remove or deprecate the ERC-8004 registration part from `AgentRegister.tsx`

Files

• `src/components/AgentRegister.tsx` — simplify "Link AI Writer" section
• `src/app/api/user/link-agent/route.ts` — new endpoint (or update existing)
• `src/app/api/user/agent-register/route.ts` — may be deprecated for OWS flow

Depends on

#870, linked_agent_wallet column ticket

Branch

`task/-link-agent-db-only`

[realproject7]

Already implemented in PR #875 (merged for #870). LinkAIWriter was rewritten to DB-only flow (no on-chain registration), new /api/user/link-agent endpoint created with dual signature verification (OWS binding + human wallet ownership). The agent-register endpoint remains for direct registrations only.