Add proper multi-root workspace support

[dollfins]

Description

The extension currently assumes a single workspace root folder. Several places use workspaceFolders?.[0] which will silently use the first folder only in multi-root workspaces, potentially scanning the wrong project.

Affected Code

1. diagnostics.ts:runGuard() — uses workspaceFolders[0] as cwd
2. diagnostics.ts:shouldScan() — uses workspaceFolders[0] for relative path calculation
3. diagnostics.ts:scanWorkspace() — iterates folders but only uses folder-local file discovery (this part is actually correct already)

Expected Behavior

When scanning a file, determine which workspace folder it belongs to and use that folder's root for:

• The CLI working directory (cwd)
• Relative path computation for exclude pattern matching

For workspace scanning, each folder should be scanned independently with results aggregated per-folder.

Implementation Notes

• Use vscode.workspace.getWorkspaceFolder(uri) to determine the correct folder for a given file
• Pass the workspace folder to runGuard() instead of always using [0]
• Consider showing per-folder scores in the report panel
• Update SorobanGuardConfig to support per-folder settings (optional, stretch goal)

Acceptance Criteria

• Files in any workspace folder are scanned correctly
• CLI runs with the correct workspace folder as cwd
• Exclude patterns apply relative to each folder's root
• Workspace scan processes all folders
• No breaking changes for single-folder workspaces

Complexity

High — requires careful refactoring of how workspace context is passed through the diagnostic pipeline.

Points

200