fix(driver-galaxy): resolve Medium code-review finding (Driver.Galaxy-009)

Fix two resource-management bugs in StartDeployWatcher / BuildDefaultHierarchySource:
(a) Replace the discarded `_ = StartAsync(...)` with an explicit task variable that
    surfaces any synchronous InvalidOperationException (called-twice guard) rather than
    silently swallowing it.
(b) Change both StartDeployWatcher and BuildDefaultHierarchySource to use ??= on
    _ownedRepositoryClient so the first client created (by whichever path runs first)
    is reused by the second path, preventing a second GalaxyRepositoryClient from being
    created and the first from leaking past the driver's lifetime.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.Galaxy/findings.md

@@ -153,13 +153,13 @@
 | Severity | Medium |
 | Category | Error handling & resilience |
 | Location | `GalaxyDriver.cs:354-371` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `StartDeployWatcher` launches the watch loop with `_ = _deployWatcher.StartAsync(CancellationToken.None)` — a fire-and-forget with a discarded `Task`. `StartAsync` can throw synchronously (`InvalidOperationException` if already started); the discard masks that programming error. Separately, `StartDeployWatcher` builds an `_ownedRepositoryClient` purely for the watcher when discovery has not run yet — if `DiscoverAsync` later runs, `BuildDefaultHierarchySource` overwrites `_ownedRepositoryClient` with a second client, leaking the first (only the latest reference is disposed in `Dispose`).
 
 **Recommendation:** Await `StartAsync` (it completes synchronously after scheduling) or at least observe its result. Reuse a single `GalaxyRepositoryClient` across the deploy watcher and the hierarchy source instead of letting `BuildDefaultHierarchySource` clobber the field — guard the assignment or build the client once in `InitializeAsync`.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — (a) replaced `_ = _deployWatcher.StartAsync(...)` discard with an explicit variable + `IsFaulted` check so any synchronous throw from `StartAsync` (e.g. called-twice `InvalidOperationException`) propagates rather than being silently swallowed; (b) changed both `StartDeployWatcher` and `BuildDefaultHierarchySource` to use `_ownedRepositoryClient ??=` so a client built by the watcher is reused by discovery instead of being overwritten and leaked — only one `GalaxyRepositoryClient` instance is now created and disposed.
 
 ### Driver.Galaxy-010