[Mem] one mem linked <identity-key> — query surface for cross-platform joins

[moekatib]

Problem

Identity keys (#128, #129, #130) are the plumbing. Without an ergonomic query surface, the feature isn't reachable — users fall back to raw `mem sql` with JSONB array operators, which most won't write.

The demo-able moment is: "one email address, one command, records from every connected platform."

Proposed

Add `one mem linked ` — list every mem record whose `keys` array contains the given value, grouped by type.

```bash
$ one mem linked email:jane@acme.com
attio/attioPeople 1 record Jane Smith (Acme Corp)
gmail/threads 14 records most recent: "re: Q2 pricing" 2d ago
google-calendar/events 3 records next: "Jane <> Moe" tomorrow 10am
fathom/meetings 1 record "Acme intro call" 2026-04-10
```

Flags:

• `--type ` filter to one record type
• `--limit ` cap per type (default 10)
• `--agent` JSON output grouped `{ type: [items] }`

Under the hood this is one query: `SELECT * FROM mem_records WHERE keys @> ARRAY[$1]`. The value-add is the grouped rendering and the agent-friendly JSON, not the SQL.

Optional second shape: `one mem linked ` for intersection — records carrying both keys (e.g. "find threads involving BOTH jane@acme.com AND bob@acme.com").

Acceptance

• `one mem linked email:jane@acme.com` groups results by type, shows count + top N.
• `one --agent mem linked ...` returns `{ key, total, byType: { : { count, items } } }`.
• Works across any prefix (`email:`, `domain:`, future `phone:`, etc.) — not hardcoded to email.
• Documented in `one guide memory` under Cross-Platform Identity.

Blocked on

#128. Useful even before #129/#130 land since Attio-only linking would still work.

Related

Cross-platform identity story. This is the command that makes the feature demo-able. Found during PR #125 testing.