Tmux wrapper: session operations

Wrapper for tmux session operations via subprocess.

## Interface

```go
type Tmux struct{}

func NewTmux() *Tmux

// Session lifecycle
func (t *Tmux) NewSession(name, workDir string) error
func (t *Tmux) KillSession(name string) error
func (t *Tmux) HasSession(name string) (bool, error)
func (t *Tmux) ListSessions() ([]string, error)

// Session interaction
func (t *Tmux) SendKeys(session, keys string) error
func (t *Tmux) CapturePane(session string, lines int) (string, error)
func (t *Tmux) AttachSession(session string) error

// Window operations (if needed)
func (t *Tmux) SelectWindow(session string, index int) error
```

## Session Naming

Convention: `gt-<rig>-<polecat>` (e.g., `gt-wyvern-Toast`)

## Implementation

- Shell out to tmux binary
- CapturePane uses `tmux capture-pane -p -t <session> -S -<lines>`
- SendKeys uses `tmux send-keys -t <session> '<keys>' Enter`

## Error Handling

- "no server running": tmux not started (not an error for HasSession)
- "session not found": session doesn't exist
- Wrap all tmux errors with context

## Environment

Requires TMUX_TMPDIR or uses default socket. Don't assume tmux is running.


Remove keepalive/heartbeat staleness thresholds

**Files:**
- keepalive/keepalive.go (lines 94-111)
- deacon/heartbeat.go (lines 88-108)

**Current behavior:**
- Hardcoded thresholds: 2min=fresh, 5min=stale, 15min=very stale
- Used by daemon to decide whether to poke agents
- Go classifies agent responsiveness

**Fix:**
- Remove all hardcoded time.Duration thresholds
- Keepalive becomes pure timestamp storage
- Deacon molecule step does health assessment:
  "Check if Witness responded in last N minutes" (N from mol config)

**Lines to remove:**
- keepalive.go:96-110 (classification logic)
- heartbeat.go:91-107 (age classification)


Merge: gt-cp2s

branch: polecat/rictus
target: main
source_issue: gt-cp2s
rig: gastown


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.