[Feature Request] Support nested capability paths in spec archive (e.g. specs/chats/my-chats/spec.md) #796

Current behavior

The archive command applies delta specs by doing a shallow readdir of changes/<name>/specs/, mapping each direct subdirectory to openspec/specs/<capability>/spec.md. This only supports one level deep:

```
changes/my-change/specs/my-chats/spec.md → specs/my-chats/spec.md ✓
changes/my-change/specs/chats/my-chats/spec.md → specs/chats/my-chats/spec.md ✗ (skipped)
```

Relevant code in specs-apply.js > findSpecUpdates:
```js
const entries = await fs.readdir(changeSpecsDir, { withFileTypes: true });
for (const entry of entries) {
if (entry.isDirectory()) {
const specFile = path.join(changeSpecsDir, entry.name, "spec.md");
const targetFile = path.join(mainSpecsDir, entry.name, "spec.md");
```

Desired behavior

Support nested capability paths so delta specs can mirror a hierarchical product structure:

```
changes/my-change/specs/chats/my-chats/spec.md → specs/chats/my-chats/spec.md ✓
changes/my-change/specs/chats/group-setup/join-requirements/spec.md → specs/chats/group-setup/join-requirements/spec.md ✓
```

Use case

We organize main specs in openspec/specs/ as a nested tree mirroring our PRD structure (e.g. specs/chats/, specs/chats/my-chats/, specs/wallet-and-tx/orders/). We want delta specs in changes to follow the same structure, so archive can merge them to the right node.

Proposed fix

Make findSpecUpdates traverse recursively: find all spec.md files under changeSpecsDir, and map each one to the corresponding path under mainSpecsDir, preserving the relative path.