Synchronized /act-on-feedback skill can be advertised while its backing file is missing

## Type

Bug

## Summary

Invoking `/act-on-feedback` in an Agent Host session can fail because the synchronized customization advertises the skill, but the corresponding `SKILL.md` file is missing from the synchronized plugin cache.

When this happens, the agent has no workflow instructions for submitted feedback attachments. In the observed case it fell back to `viewUnreviewedComments`, which returned no comments because that tool only returns comments selected through its reveal confirmation (`pendingAgentReveal`). The comments submitted with the turn were already in the `submitted` state and were available through `listComments`.

The feedback annotations and turn attachments were delivered correctly; the failure was caused by the unavailable synchronized skill followed by an incorrect fallback tool choice.

## Steps to reproduce

1. Open an Agent Host session with the synchronized `act-on-feedback` customization available.
2. Add several code-review comments and submit them to the session.
3. Invoke `/act-on-feedback`.
4. Observe that loading the skill can fail with an `ENOENT` for its cached `SKILL.md` path, despite the skill being advertised to the agent.
5. If the agent falls back to `viewUnreviewedComments`, observe that it receives an empty comment list even though `listComments` returns the submitted comments.

## Actual behavior

The skill invocation fails with an error shaped like:

```text
Failed to read skill file: ENOENT: no such file or directory, open '/agentPlugins///skills/act-on-feedback/SKILL.md'
```

Agent Host logs show the synchronized plugin being synced and stale nonces being evicted, but the advertised skill path later points to a missing file.

The submitted turn still contains annotation attachments for each feedback comment. Those attachments instruct the agent to use `listComments`, but without the skill workflow the agent may choose `viewUnreviewedComments`. That tool completes successfully with:

```json
{ "comments": [] }
```

A subsequent `listComments` call returns the submitted comments.

## Expected behavior

- A skill should not be advertised unless its backing file is readable.
- Synchronizing or evicting plugin cache entries should not invalidate paths already exposed to an active session.
- If loading a synchronized skill fails because its cache entry disappeared, the host should refresh/re-resolve the customization or report a recoverable synchronization error.
- `/act-on-feedback` should reliably use `listComments` for feedback annotations attached to the submitted turn.

## Investigation details

The event ordering was:

1. Feedback annotations transitioned from `accepted` to `submitted`.
2. The turn started with annotation attachments containing the submitted comment IDs.
3. Loading `act-on-feedback` failed with `ENOENT` under the synchronized plugin cache.
4. `viewUnreviewedComments` was called and returned no comments.
5. `listComments` later returned all submitted comments.

This is consistent with the current tool semantics:

- Submitted feedback attachments contain annotation IDs and direct the agent to `listComments`.
- `viewUnreviewedComments` returns only reviewable annotations carrying the transient `pendingAgentReveal` flag set by its confirmation UI.
- Submitted attachment comments do not carry that flag.

## Suggested areas to investigate

- Atomicity and lifetime of synchronized plugin cache directories/nonces.
- Whether skill discovery results can outlive eviction of their resolved backing paths.
- Retrying skill resolution after an `ENOENT` caused by plugin synchronization.
- A built-in fallback for feedback attachments that directs the agent to `listComments` when the customization skill cannot load.

All identifiers, filesystem paths, comment content, and account information have been omitted from this report.