Swarm learning: Beads database locality gap

## PGT Bug (GGT Design Already Correct)

Mayor created issues in `~/ai/mayor/rigs/beads/.beads/` but polecats use `~/ai/beads/.beads/`. Different databases = polecats can't see Mayor's beads.

**GGT Fix**: architecture.md already specifies:
- All agents use BEADS_DIR pointing to rig-level `.beads/`
- Lines 116-143: Beads Configuration for Multi-Agent
- Lines 573-586: Rig-Level Beads via BEADS_DIR

This is a PGT implementation gap, not a design issue. GGT spawn must set BEADS_DIR correctly.

PGT implementation gap documented. GGT design already correct - architecture.md specifies BEADS_DIR for all agents pointing to rig-level .beads/


Per-rig beads repo configuration

Add per-rig beads configuration to rig config schema.

## Config Schema

In each rig's config.json:

```json
{
  "version": 1,
  "name": "wyvern",
  "git_url": "https://github.com/steveyegge/wyvern",
  "beads": {
    "repo": "local",    // "local" | "<path>" | "<git-url>"
    "root": null,       // Override bd --root (optional)
    "prefix": "wyv"     // Issue prefix for this rig
  }
}
```

## Repo Options

| Value | Meaning | Use Case |
|-------|---------|----------|
| `"local"` | Use project's `.beads/` | Own projects, full commit access |
| `"<path>"` | Use beads at path | OSS contributions |
| `"<git-url>"` | Clone and use repo | Team shared beads |

## Environment Injection

When spawning polecats, Gas Town sets:
```bash
export BEADS_ROOT="<resolved-path>"
```

## Resolution Logic

```go
func ResolveBeadsRoot(rigConfig *RigConfig, rigPath string) (string, error) {
    beads := rigConfig.Beads
    switch {
    case beads.Root != "":
        return beads.Root, nil
    case beads.Repo == "local" || beads.Repo == "":
        return filepath.Join(rigPath, ".beads"), nil
    case strings.HasPrefix(beads.Repo, "/"):
        return beads.Repo, nil
    case strings.Contains(beads.Repo, "://"):
        return cloneAndResolve(beads.Repo)
    default:
        return filepath.Join(rigPath, beads.Repo), nil
    }
}
```

## Backwards Compatibility

If `beads` section missing, assume `"repo": "local"`.