Outpost configuration: YAML config and OutpostManager

## Overview

Configuration file for outposts and manager to load/track them.

## Config File

`~/ai/config/outposts.yaml`:

```yaml
outposts:
  - name: local
    type: local
    max_workers: 4

  - name: gce-burst
    type: ssh
    host: 10.0.0.5
    user: steve
    ssh_key: ~/.ssh/gce_worker
    town_path: /home/steve/ai
    max_workers: 8

  - name: cloudrun-burst
    type: cloudrun
    project: my-gcp-project
    region: us-central1
    service: gastown-worker
    max_workers: 20
    cost_cap_hourly: 5.00

policy:
  default_preference: [local, gce-burst, cloudrun-burst]
  overrides:
    - condition: "priority >= P3"
      prefer: cloudrun-burst
```

## OutpostManager

```go
type OutpostManager struct {
    outposts map[string]Outpost
    policy   AssignmentPolicy
}

func NewOutpostManager(configPath string) (*OutpostManager, error)
func (m *OutpostManager) Get(name string) (Outpost, bool)
func (m *OutpostManager) List() []Outpost
func (m *OutpostManager) SelectOutpost(issue Issue) Outpost
```

## Files

- `internal/outpost/manager.go`
- `internal/outpost/config.go` (extend)
- `internal/outpost/policy.go`

Depends on: gt-9a2.1 (interfaces)


request-shutdown

Send shutdown request to Witness.
Wait for termination.

The polecat is now ready to be cleaned up.
Do not exit directly - wait for Witness to kill the session.

Depends: generate-summary


Heartbeat convention: simple liveness signal for agents

Lightweight liveness signal extracted from Deacon epic (gt-5af).

**Implementation**: Each agent writes a timestamp file on activity:
```bash
echo '{"ts":"'$(date -Iseconds)'"}' > ~/gt/<role>/heartbeat.json
```

**Integration points**:
- SessionStart hook writes heartbeat
- Periodic activity (mail check, work completion) refreshes it
- `gt status` shows staleness (e.g., 'mayor: 5m ago')

**Weight**: ~5 lines per agent
**Value**: Quick debugging - see which agents are active at a glance

No monitoring daemon needed - human checks `gt status` when curious.


Molecule CLI: bd molecule commands

Add molecule commands to bd:

## Commands

```bash
bd molecule list              # List molecules (type: molecule)
bd molecule show <id>         # Show molecule with parsed steps
bd molecule parse <id>        # Validate and show parsed structure
bd molecule instantiate <mol-id> --parent=<issue-id>  # Create steps
bd molecule instances <mol-id>  # Show all instantiations
```

## gt spawn integration

```bash
gt spawn --issue <id> --molecule <mol-id>
```

This should:
1. Call `bd molecule instantiate` (creates child beads atomically)
2. Spawn polecat on first ready step
3. Polecat grinds through via `bd ready`

## Output Examples

```
$ bd molecule show mol-abc

mol-abc: Engineer in a Box
Type: molecule

Steps (5):
  design      → (ready first)
  implement   → Needs: design
  review      → Needs: implement
  test        → Needs: implement
  submit      → Needs: review, test

Instances: 3
```

```
$ bd molecule instances mol-abc

Parent          Status    Created
gt-xyz          done      2025-12-15
gt-abc          active    2025-12-17 (3/5 complete)
gt-def          pending   2025-12-18
```


Refinery-Witness communication protocol

Define mail protocol between Refinery and Witness:

FROM Witness → Refinery:
- 'Polecat ready': polecat X completed work, ready for merge
- 'Rework complete': polecat Y finished requested rework

FROM Refinery → Witness:
- 'Merge success': polecat X merged, can be cleaned up
- 'Merge failed': polecat X needs rework on <reason>
- 'Rework request': please have a polecat rebase X on current main

Implement as structured mail with parseable format.