fix: replace bare Error throws with domain-specific error classes

[ntegrals]

Summary

Replaces all 15 bare throw new Error() calls in production source code with domain-specific error classes, enabling callers to catch and handle specific failure types.

New error classes

SandboxError — file system access violations (path traversal, file not found, read-only, size limits)

class SandboxError extends OpenBrowserError {
  readonly filePath?: string;
}

BridgeError — MCP bridge failures (unknown resources, tool call errors)

class BridgeError extends OpenBrowserError {}

Changes by file

File	Throws	Error class used
sandbox/file-access.ts	8	SandboxError
viewport/viewport.ts	2	ViewportError
viewport/event-hub.ts	2	ViewportError
commands/extraction/extractor.ts	1	PageExtractionError
bridge/server.ts	1	BridgeError
bridge/client.ts	1	BridgeError
agent/instructions.ts	1	AgentError
agent/replay-recorder.ts	1	OpenBrowserError

Before / After

// Before — caller can't distinguish error types
try { await fileAccess.read('data.bin'); }
catch (e) { /* is this "not found" or "binary file" or "too large"? */ }

// After — caller can catch specific failures
try { await fileAccess.read('data.bin'); }
catch (e) {
  if (e instanceof SandboxError) {
    console.log(e.filePath); // "data.bin"
  }
}

What was NOT changed

• Test files — test mocks legitimately throw new Error() to simulate failures
• No runtime behavior changes — same messages, same throw points

Test plan

• bun run build — all 3 packages compile clean
• bun run test — all 364 tests pass
• Verified zero bare throw new Error() remaining in production code