fix: pin golangci-lint install to commit SHA (fixes #13)

- Fix Dockerfile: use correct v2 module path with commit SHA
  (github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638...)
- Add CGO_ENABLED=0 for alpine compatibility
- Fix 35 lint issues found by golangci-lint v2.1.6:
  - Remove unused nolint directives, add nolintlint where needed
  - Pre-allocate migrations slice
  - Use t.Context() instead of context.Background() in tests
  - Fix blank lines between comments and exported functions in ui.go

cmd/chat-cli/api/client.go

@@ -123,7 +123,7 @@ func (c *Client) PollMessages(
 
 	req.Header.Set("Authorization", "Bearer "+c.Token)
 
-	resp, err := client.Do(req) //nolint:gosec // URL from user config
+	resp, err := client.Do(req) //nolint:gosec,nolintlint // URL from user config
 	if err != nil {
 		return nil, err
 	}
@@ -269,7 +269,7 @@ func (c *Client) do(
 		req.Header.Set("Authorization", "Bearer "+c.Token)
 	}
 
-	resp, err := c.HTTPClient.Do(req) //nolint:gosec // URL from user config
+	resp, err := c.HTTPClient.Do(req) //nolint:gosec,nolintlint // URL from user config
 	if err != nil {
 		return nil, fmt.Errorf("http: %w", err)
 	}