# T0.4: Pi & Error Classification

## Scope
Implement `action/skills.go` with `PrepareSkillsActivity` and `classifyPiErr` for skill prep via homelab API.

## Implementation

### File: `action/skills.go`
```go
type SkillRef struct {
    Name string
    URL  string
}

type PrepareSkillsInput struct {
    Skills        []SkillRef
    StreamTimeout time.Duration
}

func PrepareSkillsActivity(ctx context.Context, in PrepareSkillsInput) error
    // For each skill, run: pi clone-or-fetch $skill.URL
    // Pass --stream-timeout=$StreamTimeout to pi
    // Each skill behind its own lock (not orchestrator.lock)
    // On error, return classified error (see below)

func classifyPiErr(err error) error
    // 4xx (400-499): NonRetryableApplicationError "PiClientError"
    // 504: ApplicationError "PiStreamTimeout"
    // 5xx (500-599, except 504): leave retryable
    // Other network errors: leave retryable
```

## Error Buckets

### Bucket 1: 4xx (PiClientError)
- Status code 400-499
- Non-retryable: bad request, auth error, not found
- Temporal stops retrying immediately
- Activity fails

### Bucket 2: 5xx except 504 (generic 5xx)
- Status code 500-503, 505-599
- Retryable: server error, likely transient
- Temporal backs off + retries until `ScheduleToCloseTimeout` (5m)

### Bucket 3: 504 (PiStreamTimeout)
- Status code 504
- Means pi's SSE stream-read timed out
- Retryable, BUT: Orchestrator doubles `config.Tuning.PiRetry.StreamTimeout` before retry
- Next attempt uses wider timeout

## Verification
```bash
cd /Users/rockliang/workplace/Poimen/workflows
go test -v ./tests -run TestPiErrors

# Test file: tests/pi_test.go
```

Test cases:
- Mock 400 response → NonRetryableApplicationError returned
- Mock 403 response → NonRetryableApplicationError returned
- Mock 500 response → retryable error returned
- Mock 503 response → retryable error returned
- Mock 504 response → ApplicationError type "PiStreamTimeout" returned
- Mock network timeout → retryable error returned

## Done Criteria
- `go test ./tests -run TestPiErrors` passes all 6 test cases
- All error buckets correctly classified
- No panics on nil pointers
- Error messages include HTTP status code