Prevent headless ACP from hanging on system keyring reads

🤖 > **Disclaimer:** This is a potential issue identified during code review. As far as we know, it has not been observed in the wild.

**Describe the bug**

With the default `system-keyring` feature, `goose acp` can hang indefinitely when a headless process triggers a system keyring prompt that cannot be answered over piped stdio. On macOS, this can occur when an unsigned process triggers a Keychain ACL prompt.

Provider construction during `session/new` reads secrets synchronously. `Config::all_secrets()` also holds its cache mutex while loading them, so a blocked keyring read can occupy a Tokio worker and block concurrent secret users.

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**To reproduce**

1. Run `goose acp` over piped stdio in a headless context that triggers a Keychain authorization prompt.
2. Send `session/new` for a provider that reads a stored secret.
3. The request waits indefinitely because the prompt cannot be answered.

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**Expected behavior**

Noninteractive ACP requests should not indefinitely block runtime workers or concurrent secret lookups. Interactive CLI and desktop users should still be able to complete system authentication without a short global timeout.

Suggested direction:

- Move ACP keyring reads off Tokio core workers.
- Avoid holding the secrets-cache mutex across the blocking read.
- If a timeout is needed, scope and configure it for noninteractive execution, fail closed with guidance to use environment credentials or `GOOSE_DISABLE_KEYRING=1`, and do not treat a timeout as a missing keyring entry.
- Avoid accumulating blocked threads, since timing out the caller does not cancel the underlying OS keyring request.

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**Verification**

Using an injectable blocking keyring read, verify that ACP returns within its configured bound, concurrent secret users are not blocked on the cache mutex, and repeated requests do not create unbounded blocked workers. Also verify that interactive keyring prompts can wait longer than three seconds.

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**Environment**

- **OS:** Primarily macOS; audit Linux and Windows behavior
- **Interface:** ACP
- **Version:** Current `main`

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).