[Bug][Mobile]: Create-workspace surfaces list no agents for a WSL-hosted repo (detectAgents sent without a distro)

## Summary

Mobile's **create-workspace** surfaces still detect agents without naming a WSL distro, so a paired client on a Windows host lists no agents for a WSL-hosted repo. This is the same defect class as #19885, which PR #19886 fixed only for the New Tab drawer; #19886 deliberately left these two call sites out of scope because they need a different distro source.

## Where

Both take the local branch and call the RPC with no params, so the host answers from its own Windows PATH:

- `mobile/src/components/use-new-workspace-execution-target.ts:88` — the hook behind `NewWorktreeModal` (`mobile/src/components/NewWorktreeModal.tsx:80`).
- `mobile/src/tasks/use-mobile-tasks-workspace-ssh-state.tsx:122` — the workspace-create draft in the tasks screen.

```ts
const response = connectionId
? await client.sendRequest('preflight.detectRemoteAgents', { connectionId })
: await client.sendRequest('preflight.detectAgents') // no target named -> host-local probe
```

## Impact

On a Windows host whose repos live under `\\wsl.localhost\\...` with the agent CLIs installed inside the distro, opening "New Workspace" on the phone shows an empty agent selection — identical to the New Tab symptom in #19885. SSH-hosted repos are unaffected (they resolve through `preflight.detectRemoteAgents`), as are non-Windows hosts, where `getPreflightWslTarget` returns `null`.

## Why it was not folded into #19886

The New Tab loader could read the distro out of the existing worktree id (`${repoId}::${path}`). These two surfaces run *before* a worktree exists, so the distro has to come from the selected target repo's path instead:

- `useNewWorkspaceExecutionTarget` currently receives only `{ client, connectionId, visible }`, so `NewWorktreeModal` would have to pass the target repo (or its path) down.
- The tasks hook already holds `workspaceCreateTargetRepo`, so its path is available in place.

## Suggested fix

The host side already accepts the parameter after #19886 (`preflight.detectAgents` / `preflight.refreshAgents` take an optional `{ wslDistro?, wslDefault? }`). The client change is to derive the distro from the target repo path with the shared `parseWslUncPath` and send it, exactly as `mobile/src/session/mobile-new-tab-agent-loader.ts` now does:

```ts
const wslDistro = repoPath ? (parseWslUncPath(repoPath)?.distro ?? null) : null
await client.sendRequest('preflight.detectAgents', wslDistro ? { wslDistro } : undefined)
```

A non-WSL path yields `null` and the request stays byte-identical to today's, so hosts that predate the parameter are unaffected.

## Related

- #19885 — the reported bug (New Tab drawer)
- #19886 — the fix for the drawer plus the RPC parameter this issue would reuse