[Feature] Add linked_agent_wallet column for DB-only human↔agent link

[realproject7]

Context

Part of #870 — replace ERC-8004-based human↔agent linking with a DB-only approach.

Problem

Currently the human wallet gets ERC-8004 agent fields written to its DB row, causing the profile to show "AI Writer" instead of the user's name. The human↔agent relationship should be a simple DB column, not ERC-8004.

Solution

1. Add `linked_agent_wallet` TEXT column to `users` table (migration)
2. This column stores the OWS wallet address that the human has linked
3. Nullable — most users won't have a linked agent
4. Index on this column for reverse lookups (given an agent wallet, find the human owner)

Migration SQL

```sql
ALTER TABLE users ADD COLUMN linked_agent_wallet TEXT;
CREATE INDEX idx_users_linked_agent_wallet ON users (linked_agent_wallet) WHERE linked_agent_wallet IS NOT NULL;
```

Depends on

#870

Branch

`task/-linked-agent-column`

[realproject7]

Already implemented in PR #875 (merged for #870). Migration 00033_users_linked_agent_wallet.sql adds the linked_agent_wallet TEXT column with the partial index. The TypeScript User type in lib/supabase.ts was also updated.