feat: support virtual workspace URL path in --config-workspace

[ifdotpy]

Summary

Allow --config-workspace to accept a direct URL path (starting with /) in addition to logical cluster paths. This enables init-agent to watch InitTarget/InitTemplate resources via an APIExport virtual workspace.

Changes

• internal/kcp/rest.go: Add RetargetRestConfigToPath helper for direct URL path retargeting. Fix StripCluster to handle trailing slashes before regex match.
• cmd/init-agent/main.go: Use RetargetRestConfigToPath when config workspace starts with /. Preserve base config for leader election to avoid sending lease requests to virtual workspace URLs.
• cmd/init-agent/options.go: Update flag description and comment.
• internal/kcp/rest_test.go: Table-driven tests for RetargetRestConfig, RetargetRestConfigToPath, and StripCluster.

How it works

When the value starts with /, it is appended directly to the base KCP host URL. Otherwise, existing behavior is preserved (/clusters/<path>).

The ClusterClient (used for fetching WorkspaceTypes and InitTemplates) is initialized separately from the manager config, so it continues to use the base KCP URL regardless of the config workspace format.

Leader election uses a copy of the pre-retarget config, so lease operations always target the original KCP workspace.

Backward compatibility

Fully backward compatible. Existing --config-workspace=root:init-agent continues to work unchanged.

Fixes #18

```release-note
Support virtual workspace URL path in --config-workspace flag for self-service provider initialization.
```

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