Fix: `MaintainerActive` Signal Is Never Set — Repos Always Lose 20 Points

Nobody has claimed this yet.

The scoring engine (`engine.go`) awards **20 points** when the `MaintainerActive` signal is `true`. However, looking at `signals.go`, the `MaintainerActive` field is **never assigned** — it stays at its zero value (`false`) for every single repository.

This means every repository recommendation silently loses 20 points regardless of actual maintainer activity, making scores consistently lower than intended.

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### 📍 File to Change

`backend/core_service/internal/orchestration/signals.go`

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### 🔍 Current Buggy Code (lines ~10–40 in `signals.go`)

```go
// MaintainerActive is NEVER set — always stays false
func (s *Service) BuildRepoSignals(...) scoring.RepoSignals {
signals := scoring.RepoSignals{}

repoData, err := s.githubClient.FetchRepo(ctx, repo)
// ... MaintainerActive is never touched here

return signals
}
```

---

### ✅ What To Do

After fetching `repoData`, add a heuristic check: if the repo was pushed to within the last **7 days**, consider the maintainer active.

```go
// Add this block after the RecentActivity check in signals.go:

// Check if maintainer is active (pushed within last 7 days)
pushedTime, err := time.Parse(time.RFC3339, repoData.LastPushedAt)
if err == nil && time.Since(pushedTime).Hours() < 24*7 {
signals.MaintainerActive = true
}
```

Add a brief comment explaining the heuristic so future contributors understand the logic.

---

### 🏁 Acceptance Criteria

- [ ] `signals.MaintainerActive` is now set to `true` when the repo was pushed to within the last 7 days
- [ ] The existing `signals.RecentActivity` check (30-day threshold) is untouched
- [ ] The code compiles: `go build ./...` from `backend/core_service` must pass
- [ ] A short comment documents the heuristic (e.g., `// Active maintainer: pushed within last 7 days`)

---

### 💡 Technical Hints

- Edit `backend/core_service/internal/orchestration/signals.go` — specifically the `BuildRepoSignals` function
- `repoData.LastPushedAt` is a string formatted as RFC3339 — parse it using `time.Parse(time.RFC3339, ...)`
- The same parsing pattern is already used a few lines above for `RecentActivity` — follow the same style
- You do **not** need to make any new API calls; `repoData` is already fetched

---

### 🚀 Getting Started

1. Fork the repository
2. Create a branch: `git checkout -b fix/issue-3-maintainer-active-signal`
3. Edit `backend/core_service/internal/orchestration/signals.go`
4. Verify it compiles: `cd backend/core_service && go build ./...`
5. Open a Pull Request!

Open backend/core_service/internal/orchestration/signals.go and start with the BuildRepoSignals function, comparing the existing RecentActivity parsing with the requested heuristic. Confirm the MaintainerActive behavior and preserve the existing RecentActivity check, then run go build ./... from backend/core_service.】【。

Written by the indexing model from the issue text.