No way to view, manage, or opt out of stored memories

### Describe the bug

Memories stored via `store_memory` are scoped to the repository, not the user. There is currently no way for individual users to view, delete, or opt out of them.

## The problem at scale

In large repos with many contributors, memories accumulate quickly. Our repo has hundreds of stored memories — many low-quality, duplicated, or originating from other users' sessions. As a user, I can see that some of these memories are surfaced in my sessions automatically, but I have no visibility into how they're selected, ranked, or shared.

We've actively seen model performance degrade due to low-quality and contradictory memories. An audit of our repo's ~310 memories found that **only ~65% are accurate — meaning roughly 1 in 3 memories are wrong, duplicated, or misleading:**

| Category | Count (est.) | % |
|---|---|---|
| ❌ Inaccurate | ~30 | 10% |
| ⚠️ Partially Accurate | ~20 | 6% |
| 🔁 Redundant Duplicates | ~60 | 19% |

Key findings:
- **~50 duplicate memories** for a single rule ("never use `as` type casts"), ~15 of which **incorrectly state the rule** by prohibiting `as const` when it is explicitly allowed
- **Mutually contradictory memories** about git workflows (e.g. "always push without asking" vs "never auto-push") — none sourced from actual repo instructions
- **One dangerous memory** instructing the agent to "run shell commands immediately without discussion"
- **Fabricated facts** like hardcoded app IDs that are actually repository variables

## Impact

- A 35% miss rate on system prompt instructions is unacceptable — it actively degrades model output
- Contradictory memories force the model to arbitrate between conflicting instructions
- Repo owners *can* delete memories, but manual cleanup doesn't scale — new low-quality memories are constantly added
- The only user-level workaround is "store a correcting fact," which just adds more entries

## What's needed

1. **User-level controls:** View, delete, and opt out of memories (e.g. `/memory list`, `/memory delete`, `/memory off`)
2. **Better repo-level tooling:** Bulk management, dedup, and quality controls — not just the ability to delete one at a time
3. **Better defaults:** Automatic deduplication, expiry, quality gates, or a cap to prevent unbounded accumulation

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_