detectInstalledProviders truthy gate misclassifies subprocess slots as HTTP #174

Context

bin/manage-agents-core.cjs:362:

```js
if (!p.cli || p.type === 'http') return true;
```

The intent is "HTTP-only slots are always included (no CLI to probe)". The implementation also returns true for any subprocess slot where p.cli is falsy — which is the default state after ensureMcpSlotsFromProviders reconstructs providers.json from ~/.claude.json mcpServers (those entries only set mainTool, not cli).

Consequence: subprocess slots with cli: null skip the binary-existence check entirely. They get included even when the underlying CLI isn't installed, producing UNAVAIL at dispatch time instead of being filtered out at startup with a clear "CLI not found" message.

Proposed solution

Replace the truthy check with an explicit type check:

```js
if (p.type === 'http') return true;
const binaryPath = p.resolvedCli || p.cli || p.mainTool;
if (!binaryPath) {
process.stderr.write([manage-agents-core] ${p.name} has no resolvable CLI (cli, resolvedCli, mainTool all empty) — excluded\n);
return false;
}
// ... existing accessSync probe ...
```

Combined with PR #161's mainTool fallback in resolveCli, this gives the function a consistent contract: every subprocess slot must have some resolvable binary or it gets excluded with a clear log line.

Acceptance criteria

• HTTP slots always pass (current behavior preserved)
• Subprocess slots with no resolvable binary excluded with a diagnostic
• Subprocess slots with cli: null but valid mainTool correctly probed via mainTool
• Existing tests still pass