[Feature] Update profile display to use linked_agent_wallet

[realproject7]

Context

Part of #870 — fix profile display after architecture change.

Problem

After moving ERC-8004 to OWS wallet and human↔agent link to DB-only:

• Human profile should show "Project7" + "Operates: AI Writer" (via `linked_agent_wallet`)
• Agent profile should show "AI Writer" + "Operated by: Project7" (reverse lookup via `linked_agent_wallet`)

Solution

Human profile (`0x4d68...`)

• `agentMeta` will be `null` (no ERC-8004 on human wallet) → `isAgent = false` ✓
• Check `dbUser.linked_agent_wallet` — if set, show "Operates: AI Writer #{id}" section
• Fetch agent info from ERC-8004 via the linked wallet address

Agent profile (`0xdABF...`)

• `agentMeta` will be non-null (ERC-8004 registered) → `isAgent = true` ✓
• Find human owner: query `users` where `linked_agent_wallet = 0xdABF...` → get human's FID
• Show "Operated by: {name}" with Farcaster avatar

WriterIdentity component

• Update `getAgentOwnerProfile()` to use `linked_agent_wallet` reverse lookup instead of `agent_owner` field
• Simplify `isAgentOwner` logic — if wallet has no ERC-8004, it's never an agent

Files

• `lib/actions.ts` — `getFullUserProfile()`, `getAgentOwnerProfile()`, `fetchAgentMetadata()`
• `src/app/profile/[address]/page.tsx` — "Operates" section for human profiles
• `src/components/WriterIdentity.tsx` — owner lookup via `linked_agent_wallet`

Depends on

#870, linked_agent_wallet column, link-agent-db-only

Branch

`task/-profile-linked-agent`

[realproject7]

Already implemented in PR #875 (merged for #870). Profile display updated: getFullUserProfile returns linkedAgentMeta for human owners, getAgentOwnerProfile does reverse lookup via linked_agent_wallet, profile page shows separate 'Linked AI Writer' card for human owners and 'Operated by' for agent profiles. isAgentOwner logic uses linked_agent_wallet.